Is Christianity Shituf or Avodah Zarah?
Rabbi Moshe Sternbuch, chief of the Edah HaChareidis rules that Christian denominations that believe Yeshu ימש״ו is the incarnation of G-d chas v’shalom, fall under the category of actual Avodah Zarah and are not considered Shituf/"partnership" (link - link).
“Christianity is not merely Shituf, which according to some opinions gentiles aren’t warned against, in truth they corporealize our G-d blessed be He, through their idolatry with the title “son” ר״ל, the gravest form of idolatry." (Teshuvos VeHanagos siman 317)
(Since there is a prohibition against learning the ways of Avodah Zarah (Rambam, Avodah Zarah 2:2, Shulchan Aruch, Yoreh Deah 139) don’t open the links below without consulting a Rov.)
There is a common misconception amongst Yidden regarding mainstream Christianity, i.e. trinitarian Christianity. Their deity is not a sort of tritheism with G-d at its head. That would actually constitute heresy in trinitarian Churches. The Trinity is a “homoousion”, the belief that G-d is three distinct hypostases or persons who are coeternal, coequal, and indivisibly united in one being, or essence. In fact, that is why (link - link) they falsely claim to be monotheistic. In order to reconcile their three G-ds with the one G-d of the Torah they just combined them into one and called it a paradox. Even according to the opinions based on Tosafos to Bechoros 2b Sanhedrin 63b that shituf(1) is permitted for Bnei Noach, this doctrine should be actual Avodah Zarah. They don’t believe in some sort of partnership of false deities with the Creator or as intermediaries between them and Him. They redefined Him as composed of three primordial equal parts chas v’shalom, one even having form, i.e. Yoshke ימש"ו.
(1)One clear example of the definition of Shituf: The Rama OC 156 quotes Tosafos 63b “sh’mishtatfin shem shamayim v’davar acher”, the Olas Tomid on this Rama says:
“klomar sh’omrim d’ze l’mala mize (they recognize Hashem is above they’re AZ) veinam pogmim b’yichud Hashem (and they don't compromise Hashem's unity), omnam im pogmim b’yichud v’omrim sh’shnayim hem, ze bichlal AZ”.
Meaning if they believe it’s literally a partnership then it would be AZ (how much more so if they believe they’re one essence). Others limit shituf even further (See Sefer Hamamarim Melukat 1 p.323, Mamar Mayim Rabim 5717, Melukat 3 p. 128 B’yom Ashtei Asar 5731 ibid footnote 20)
In other words, they would have been better off believing in a sort of tritheism than redefining G-d in this preposterous form (although as quoted above from Rav Sternbuch, by claiming Yoshke is the incarnation of G-d and not merely another deity, they would anyway be corporealizing Him). Shituf is permitted according to some opinions, but only as long as they also believe in Hashem (as the Torah defines Him). How is this doctrine any different from that of the Greek g-ds with Zeus as the chief deity of the pantheon with Yoshke instead? In conclusion, the Trinity is the Christian version of the pagan pantheons of antiquity and is pure Avodah Zarah.
It should be noted though, the Lubavitcher Rebbe in a handwritten editing remarks to a letter
“Christianity is Avoda Zara, is in contrast to the seven Nohadite laws, as opposed to Islam. However, the Christians of today are simply “Maaseh Avoseihem Beyadeihem”.
Accordingly, the average layman may not be considered an idolator.
Survey - A Majority Of Americans Think J Is A Great Teacher Yet Reject His Claims To Be G-d
Halachic Ruling by Rav Sternbuch
(Nontrinitarianism rejects the doctrine of the Trinity. The din of such churches depends on their respective beliefs)
The Prohibition of Idol Worship
From The Divine Code by Moshe Weiner
The Master of the universe commanded Adam in the prohibition against serving idols, as it says, “And the Lord God commanded [upon] Adam …,” meaning that God commanded Adam to submit to His Divinity. The Sages explained that there are three meanings in this: “I am God; do not exchange Me” – to rebel and replace Me with another god, which is the prohibition of idolatry. “I am God; do not curse Me” – this being the prohibition of blaspheming God’s Name, since for God’s honor one must not disgrace and blaspheme Him. “I am God; the fear of Me shall be upon you” – this being the obligation to fear God. The prohibition of idolatry has two facets: the command to recognize and know God, and the prohibition against serving idols. Anyone who does not recognize and believe in God is a “deviant believer” (see Part I, topic 1:7). Likewise, anyone who serves idols denies all of God’s commandments (since he does not accept God’s Sovereignty), as well as His honor and His True Existence.
The main prohibition against idol worship is not to serve one of the creations, be it an angel, a spiritual power, a constellation, a star or a planet, one of the fundamentals of the physical creation, a person, an animal, a tree, or any other created thing. Even if one knows that the Master of the universe is God, and he only serves a lofty creation and only in the mistaken manner that Enosh and his generation did (Gen. 4:26, as will be explained below in topic 4), this is still idol worship. This concept is written in the Torah: “Lest you raise your eyes to the heavens and see the sun and moon and the stars and the hosts of the heavens, and you are persuaded, and you bow down to them and serve them – those [celestial bodies] that God separated for all the nations under the heavens.” This means that a person is able to err in his heart and believe that celestial bodies guide the world, and that God has chosen them to be forever alive and existing without ever decaying, for the sake of the terrestrial world but unlike its way. From this false idea, one may come to think that it is fitting to bow down and serve them. Regarding this it says, “Guard yourselves lest your hearts stray,” meaning: be vigilant to avoid a mistaken thought, by imagining that these creations are acting as intermediaries between people and God. Therefore, a person is also an idol worshiper if he serves God along with another entity as an intermediary (see Part I, topic 1:7), even if he says that the Lord is the “main God,” but he also serves another power. This is so regardless of whether one serves the intermediary alone, for example by bringing a sacrificial offering or bowing down to it, or if he brings a sacrifice and bows down and says that his service is for both God and the intermediary. However, if a person serves only the Lord, but he also believes there is another power or god under the Lord’s command that one should also have faith in and swear by, then he is called a “believer in an intermediary.” Great Rabbinical authorities throughout history debated whether the false belief in an intermediary is actually idolatry and therefore prohibited to Gentiles, or if it is not included in the basic Noahide prohibition of idol worship and therefore not forbidden for Gentiles., , 
Idol worship does not only include the worship of an angel,
a physical creation, or some natural or metaphysical power. If one
accepts upon himself any created or imagined entity, spiritual or
physical, as a deity, and he worships it and totally subdues himself
to it, as a servant before his master, this is idol worship. For
example, those who worship spirits of the dead or any other spirits
which they imagine to exist, are idol worshipers. Likewise, this
applies if one worships any ideal that was imagined by some people
to be a motivating reason for the universe, if he serves this spirit
or ideal in the manner of those who bow down or bring incense to the
ideals of “peace,” “love,” or “humanitarian rights.” The basic idea
is that one who actually serves any part of the physical or
spiritual creation (which includes everything except God Himself,
Who is not created) is an idol worshiper. One should know that all
these are natural created things, made for the sake of mankind, to
help him in his service to God, and they were not created so that
people should make them rulers over themselves. Those who exchange
the secondary with the fundamental are transgressing the command
(explained above in topic 1), “I am God; do not exchange Me.”
What was the mistake of Enosh and his generation? In his days,
mankind made a great mistake, and the wise men of that generation
gave thoughtless and spiritually erroneous advice. They said that
since God created the stars and the planets with which to control
the world, and He put them in the heavens and treated them with
honor, making them servants who minister before Him, it is therefore
proper to praise them, glorify them, and treat them with honor.
These people also said that it is the will of God that mankind
should honor and make great those whom He magnified and honored,
just as a king desires that the servants who stand before him will
be honored, for doing so is an expression of honor to the king. Once
they thought this, they began to build places of worship for the
stars and to offer sacrifices to them. They would praise and glorify
them with words, and prostrate themselves before them, because by
doing so, they would – according to their false conception – be
fulfilling the will of God. This was the essence of the worship of
false gods, and this was the reasoning of those who worshiped them,
and the explanation they gave. They did not say that there is no
other god except for this star they were worshiping. This is what
Jeremiah conveyed: “Who would not fear You, O King of the
nations? For [kingship] benefits You, for among all the wise men of
the nations and in all their kingdoms, [it is known that] there is
none like You. But in one concept they are foolish and stupid; the
vain [idols] which they teach are but wood.” This means that all
people knew that God alone exists, but it was from their mistake and
their foolishness that they said that this vanity of theirs (the
concept of independent intermediaries and the worship of idols) was
After many years passed, there arose false prophets
who said that God had commanded them to say to the people: Serve
this star (or all the stars); sacrifice to it and offer libations to
it, and build a temple for it and make an image of it, so that
everyone – including the women, the children, and the general
population – could bow to it. A false prophet would inform them of a
form that he had conceived, and tell them that this is the image of
the particular star, claiming that this was revealed to him in a
prophetic vision. In this manner, the people began to make images in
temples, under trees, and on the tops of mountains and hills. People
would gather together and bow down to the images, and the false
prophets would say: “This image is the source of benefit or harm. It
is appropriate to serve it and fear it.” Their priests would tell
them: “This service will enable you to multiply and be successful.
Do this and this, or do not do this and this.” Subsequently, other
deceivers arose and declared that a specific star, sphere or angel
had spoken to them and commanded them: “Serve me in this
manner.” The false prophet would then relate a mode of service,
telling them: “Do this, and do not do this.” Thus, these practices
spread throughout the world. People would serve images with strange
practices – one more distorted than the other – offer sacrifices to
them, and bow down to them. As the years passed, God’s glorious and
awesome Name was forgotten by the entire population. It was no
longer part of their speech or thought, and they no longer knew Him.
Therefore, all the common and uneducated people and their children
eventually knew only the images of wood or stone which they were
trained from their childhood to bow down to and serve, and in whose
name they swore, and in whose temples they worshiped. The wise men
among them would think that there was no God other than the stars
and spheres for whose sake, and in resemblance of which, they had
made these images. The True God was not recognized or known by
anyone in the world, with the exception of a few individuals: for
example, Enoĥ, Methuselaĥ, Noaĥ, Shem and Eber. The world continued
in this fashion until the pillar of the world –Abraham the
Hebrew – was born.
After this mighty man was weaned, he began to explore and think. Though he was a child, he began to think incessantly throughout the day and night, wondering: “How is it possible for the celestial firmament to continue to revolve without having anyone controlling it? Who is causing it to revolve? Surely it does not cause itself to revolve!” He had no teacher, nor was there anyone to inform him. Rather, he was mired in Ur Kasdim among the foolish idolaters. His father, mother, and all the people around him were idol worshipers, and he would worship with them. However, his heart was exploring and gaining understanding. Ultimately, he appreciated the way of truth and understood the path of righteousness through his accurate comprehension. He realized that there is One God who controls the celestial sphere and Who created everything, and that there is no other God among all the other entities. He knew that the entire world was making a mistake in worshiping creations. What caused them to err was their service of the stars and images, which made them lose awareness of the truth. Abraham was forty years old when he became fully aware of his Creator. When he recognized and knew Him, he began to formulate the replies to the inhabitants of Ur Kasdim and debate with them, telling them that they were not following a proper path. He broke their idols and began to teach the people that it is fitting to serve only the God of the universe, and to Him alone is it fitting to bow down, sacrifice, and offer libations, so that the people of future generations would learn to recognize Him. Conversely, he realized that it is fitting to destroy and break all idolatrous images, lest people err and think that there is no One God, but rather only these images. When he overcame them through the strength of his arguments, the king, Nimrod, desired to kill him, but he was saved through a miracle, and he left for Ĥaran. There, he began to call in a loud voice to all the people and inform that there is one God in the entire world, and it is proper to serve only Him. He would go out and call to the people, gathering them in city after city and country after country, until he finally arrived in the land of Canaan – proclaiming God’s true existence the entire time – as it states (Genesis 21:33): “and there he proclaimed the Name of God, God of the universe.” When the people would gather around him and ask him about his statements, he would explain them to each individual according to the person’s understanding, until they turned to the path of truth. Ultimately, thousands and tens of thousands gathered around him. He planted in their hearts this great fundamental principle, and he composed texts about it.
 Tractate Sanhedrin 56b; Sifri Numbers 15:23; Rambam, Laws of Kings 9:1.  Genesis 2:16.  Deuteronomy 4:19.  Deuteronomy 11:16.  Rambam, Laws of the Worship of Stars [and Idols] 2:1, explains the main concept of idol worship as the mistaken thought that it is God’s will that we serve idols – unlike those who deny God’s existence and imagine an idol as a deity. This is also the opinion of Ramban; see his explanations on Ex. 20:3, 22:19, and 23:25. Rashi (on Ex. 20:3) and some other Torah authorities maintain that the essence of idol worship is the physical action of worshiping an idol, rather than believing in the mistaken concept. This disagreement affects the question regarding belief in intermediaries, because according to Rambam this is the main thing that is prohibited as idol worship, and Gentiles are definitely forbidden to maintain this false belief. But Rashi holds this isn’t the main prohibition, and from this follows the opinions of Tosafot (see below) that belief in an intermediary isn’t forbidden for Gentiles.  Ĥiddushei HaRan Sanhedrin 61b. * The majority opinion and practical ruling is that it is not forbidden; see topics 12:9-10 below. But it is unrighteous, i.e., the person is not one of the Pious of the nations of the world (see Part I, topic 1:7 and footnotes there).  It is clear from Rambam’s intentions (in Laws of the Worship of Stars [and Idols] 1:2, Laws of Repentance 3:7, and elsewhere) that this belief in an intermediary power is also the essence of idol worship. In Laws of Kings 9:2, he concludes and writes that any act of idol worship that is a capital sin for a Jew is also a capital sin for a Gentile. However, Rema on Oraĥ Ĥayim ch. 156 rules that the false belief in an intermediary is permitted for Gentiles, and in his Darĥei Moshe (ibid.) it is explained in detail in regard to traditional Christian doctrine, as quoted here: It is written in Sefer Toldot Adam V’Ĥavah by Rabbenu Yeruĥam: Rabbenu Yitzhak writes that it is permitted to accept an oath from a worshiper of Yeshua who swears in his religion …, if he swears by “God” and does not mention the name of Yeshua, even if he mentions God’s name in the oath with the intention of including Yeshua, because he still includes this with an intention for the Creator of Heaven and Earth. Even if he connects the name of God with his belief in the other power, nevertheless, it is not prohibited for one to cause a Gentile to mention his belief in an intermediary (even in an oath). Also the prohibition of “placing a stumbling block before the blind” does not apply, since it is not forbidden for Gentiles to believe in an intermediary. This is also written in Tosafot on Tractate Beĥorot 2b. This is also written in Tosafot on Tractate Sanhedrin 63b and Piske ha’Rosh Sanhedrin ch. 7. The implication is that the concept of a divine trinity is actual idol worship, and swearing verbally in the names of God and another power would be forbidden for Gentiles. But if they mention only God, without saying clearly that they (also) mean Yeshua (or souls of others whom the person considers to be saints), it is considered in practical terms as combining God’s power with an intermediary in an oath. That belief, and the mention of it, are not prohibited to Gentiles, and therefore if a Jew requires that a Christian should make an oath in God’s name (only), it is permitted. The explanation above (from Tosafot and Rema) is accepted by most later Rabbinical authorities (e.g., Nodah Bi’Yehudah vol 2, ch. 148). However, if one bows down before a physical statue or image, it is an act of idol worship. This distinction between belief and actual worship is clear in the following section quoted from Sefer Mitzvot Gadol (SMa״G), Prohibition 1: One may not bring to mind the thought that there is any god besides the Lord, as it says [in the Ten Commandments, Exodus 20:3], “There shall not be for you other gods before Me,” from which we see that the Torah forbids the taking of an oath in the name of an idol, and this includes belief in an intermediary, even if one doesn’t actually serve it. In Meĥilta it says: “The inference that is clear from the verse [Exodus 20:4], ‘You shall not make for yourself a carved image …,’ is not to create one. From where do I know that one should not keep in existence one that exists already? … However, the verse’s interpretation cannot be taken out of the simple meaning of the text.” Furthermore, these words …, “before Me” (i.e., “in My Presence”), are interpreted simply as the combining of God’s name with the name of another. In Tractate Sanhedrin 63a it says that one who combines God’s name with that of another will be “uprooted from the world,” as it says (Exodus 22:19), “One who slaughters [sacrifices] to the gods will be destroyed – [this is allowed] only to the Lord alone.” The explanation of his words is that Meĥilta explains this verse, “There shall not be for you other gods before Me,” to mean that one may not keep an idol in existence. However, the simple meaning of the verse is not referring to this. (Rather, the prohibition of keeping an idol in existence is learned from the verse, “You shall not bring an abomination into your house,” as it is written by Rambam in Laws of the Worship of Stars [and Idols] 7:2). The simple meaning of the verse, “There shall not be for you other gods before Me,” is that one may not have idols (“other gods”) in any way, even in a way of an intermediary. He brings proof from Tractate Sanhedrin (ibid.), which explains that the verse cited above, “One who brings offerings to the gods will be destroyed – [sacrificing is allowed] only to the Lord alone,” is referring to a punishment specifically for one who sacrifices or does some other act of idol worship. However, the prohibition itself applies even for one who does not actively serve idols, but accepts idol worship or swears by it. We see from this that the concept of an intermediary refers to believing that there is a partner along with God, as expressed either in one’s thoughts or speech, and this does not necessarily need to be an active service.  (Continuation of the preceding footnote) This is the belief in an intermediary that (as Tosafot writes) is not prohibited to Gentiles, meaning to believe and swear in both God’s name and another’s name. However, it is not permitted for a Gentile to perform an active service of idol worship. It is also clear from Rema in Yoreh De’ah ch. 141, that it is prohibited to benefit from even a plain crucifix icon unless it is nullified (see Chapter 8 below on nullification of idolatrous images, and the permissibility of a plain cross if it is only for decoration). Why would it be prohibited to benefit from this, if it is not an idol that is forbidden to Gentiles? Therefore it obviously involves actual idol worship, and the object needs to be nullified before it can be used for any benefit. Therefore there is no contradiction in the words of Rema, who writes (Oraĥ Ĥayim ch. 156) on belief in an intermediary; rather it is as explained above, that an intermediary is permitted for a Gentile only in belief and speech, but not in actual worship or service. The conclusion of Ĥatam Sofer (on Shulĥan Aruĥ Oraĥ Ĥayim ch. 156) is, “The main opinion is that a Gentile is liable for worshiping an intermediary.”  Jeremiah 10:7-8.  *In Guide for the Perplexed, Rambam explains that the stars and other celestial spheres influence our world, but they are also God’s creations and have no free will of their own. Thus, they are no more than an “axe in the hands of a woodchopper,” and should not be worshipped or served.  *The word “Hebrew” (Genesis 14:13) identified him as a descendant of the prophet Eber (see Genesis 10:25); alternatively, it literally means “from over,” since he came to the land of Canaan from over the Euphrates River.  *Abraham traveled from Ĥaran with his wife Sarah and “the souls they made in Ĥaran” (Genesis 12:5) – meaning the great number of people whose souls they had uplifted to righteousness, by teaching them to abandon idolatry and to accept the One God and His Seven Noahide Commandments. The verse Genesis 21:33 can alternatively be understood to mean that not only did Abraham call in the name of the Lord, but he also motivated others to do so, as explained in Tractate Sotah 10a. See Part I, topic 6:6. Abraham proclaimed that God’s name is “E-l olom” (“God universe”), to stress that there is no true separation between God and the universe. It is only an emanation of God’s power, which is united with God Himself in total unity. With this realization, a faithful person will be motivated from love and awe of God to serve Him and do His will. See Likkutei Siĥot vol. 7 (Vayak’hel).  Topics 4-6 are quoted from Rambam, Laws of the Worship of Stars 1:1-3. *From the time of Abraham’s passing until the giving of the Torah at Mount Sinai, aside from the family of his grandson Jacob, we only find mention of righteous individuals, rather than entire communities. It seems that for most of the thousands of “people of the house of Abraham” whom he and his wife attracted, the couple’s exceptional kindness only temporarily inspired them.
Another point worthwhile of discussion is whether the Christian doctrine of the abrogation of the “Old Testament” by the “New Testament” ח"ו disqualifies adherents of Christianity as Pious Amongst the Nations. It is a prerequisite of the 7 Noahide Laws that Gentiles abide by them because it says so in “the Torah of Moses”. (Rambam, Hilchos Melachim 8:11. Likkutei Sichos vol. 26 p. 132.) However, that is beyond the scope of this paper (possible answer).
(audio shiur on shituf based on Tosafos to Sanhedrin 63b)
ספר שבע מצות השם
אם גוים נצטוו על איסור שיתוף
האמונות והדעות לרבנו רב סעדיה גאון
תשובות והנהגות להרב משה שטרנבוך
האם הנוצרים בימינו עובדי עבודה זרה הם?
יחס היהדות לנצרות
Is Christianity considered Avoda Zarah?
(History of Trinitarian Doctrines)