What is the number one sin?

Question

What is the ultimate sin?

What I found on the web, were the big three, Incest, Murder, Idolatry.

Though what makes these three cardinal?

What about Lashon Hara, Motzey Shemra, Masturbation (The only sin that a child would be liable for if they did it at a young age), Desecration of Shabbat, Not putting on Teffilin (That's deoraita like many that are on the list), Stealing, because you're taking part of a persons soul.

Here you go, Chilul Hashem, as there's no teshuvah until death.

You can say the cardinal three, though I would disagree as they are very simply dismissed with teshuvah. As mentioned before, there are sins that are far worse, and I know there are some that go beyond to what I have written, (Willfully rejecting a brit milah, and not fulfilling Korban pesach on your table)

So why, "Incest, Murder, Idolatry"?

Answer 1

Concerning your question on why these are the three cardinal sins, the Gemara learns us the following (Arakhin 15b):

With regard to forbidden sexual relations it is written that when Potiphar’s wife attempted to seduce Joseph he responded: “How can I do this great wickedness, and sin against G-d” (Genesis 39:9). With regard to bloodshed it is written, after Cain murdered his brother: “And Cain said to the L-rd: My punishment is greater than I can bear” (Genesis 4:13). The Torah describes each of these three cardinal sins with the word “great” in the singular, whereas malicious speech is described with the plural term “great things,” indicating that it is equivalent to all three of the other transgressions together.

So, the Gemara is teaching us that the Torah uses the word "great" in describing these sins.I'll demonstrate it in the posuks below:

Incest:With regard to forbidden sexual relations it is written that when Potiphar’s wife attempted to seduce Joseph he responded: “How can I do this great wickedness, and sin against G-d” (Genesis 39:9)

Murder: With regard to bloodshed it is written, after Cain murdered his brother: “And Cain said to the L-rd: My punishment is greater than I can bear” (Genesis 4:13).

Idolatry: And it is written with regard to idol worship: “And Moses returned to the Lord, and said: Oh, this people have sinned a great sin, and have made for themselves a god of gold” (Exodus 32:31).

Concerning lashon hara, the Gemara explains further:

The Gemara asks: Granted that with regard to malicious speech the verse uses the plural: “Great things,” but the plural indicates a minimum of two. If so, one can only say that malicious speech is equivalent to two of the cardinal transgressions. The Gemara responds: Which of them could be taken out as less than the other two? All three are equal. Therefore malicious speech must be equivalent to all three.

The base for lashon hara in this Gemara is found in a posuk in Tehillim:

“May the L-rd cut off all flattering lips, the tongue that speaks great things” (Psalms 12:4).

Extra sources: תלמוד בבלי, see also footnote 31-32 in sefer הקדמת הרוקח:

אולי עפ"י מדרש תהלים מזמור יב יכרת ה' כל שפתי חלקות לשון מדברת גדולות תהילים יב ד בר קפרא בשם רבי יונתן אמר קשה לשון הרע שכבר בא דוד וכרתו ברוח הקודש שנאמר יכרת ה' כל שפתי חלקות קשה לשון הרע משלש עבירות שנקראו גדולות עבודה זרה וגלוי עריות ושפיכות דמים בעבודה זרה כתיב אנא חטא העם הזה חטאה גדולה שמות לב לא בגילוי עריות כתיב ואיך אעשה הרעה הגדולה הזאת בראשית לט ט בשפיכות דמים כתיב גדול עוני מנשוא בראשית ד יג ובלשון הרע כתיב יכרת ה' כל שפתי חלקות לשון מדברת גדולות"' אולי עפ"י אבות ג יג רבי עקיבא אומר שחוק וקלות ראש מרגילין

See also: Shaarei Teshuva 3:201-209

But why?

The Gemara elsewhere (Sanhedrin 74a) explains:

With regard to all other transgressions in the Torah, if a person is told: Transgress this prohibition and you will not be killed, he may transgress that prohibition and not be killed, because the preserving of his own life overrides all of the Torah’s prohibitions. This is the halakha concerning all prohibitions except for those of idol worship, forbidden sexual relations, and bloodshed. Concerning those prohibitions, one must allow himself to be killed rather than transgress them.

The reason why there are three cardinal sins, and these are incest, murder and idolatry, I would like to begin explaining, by bringing a Rambam.

In his Mishneh Torah; Hilchos Yesodei HaTorah 5:5 he explains:

Similarly, if gentiles told [a group of Jews]: "Give us one of you to kill. If not, we will kill all of you," they should allow themselves all to be killed rather than give over a single soul to [the gentiles].

Concerning forbidden sexual relationships, the Torah compares this to actually murdering someone:

But if a man find a betrothed girl in the field, and the man force her, and lie with her: then only the man that lay with her shall die: See Yoma 86a

Concerning idolatry, during Shema, we read "And thou shalt love the L-rd thy G-d with all thy heart, and with all thy soul, and with all thy might." (Devarim 6:5).

On this posuk in Devarim 6:5, the Gemara in Yoma 86a explains that "with all thy soul" (as we read in the Shema), actually means that:

Rather, it is to teach that if there is a person whose body is more beloved to him than his property, therefore it is stated: “With all your soul.” The verse teaches that one must be willing to sacrifice his life to sanctify God’s name.

Answer 2

You ask "What is the ultimate sin?" There isn't one. The commandments are not ranked. The Mishnah says:

וֶהֱוֵי זָהִיר בְּמִצְוָה קַלָּה כְבַחֲמוּרָה, שֶׁאֵין אַתָּה יוֹדֵעַ מַתַּן שְׂכָרָן שֶׁל מִצְוֹת. רַבִּי אוֹמֵר Rabbi [Yehudah haNasi] said: Be as scrupulous in observing a [seemingly] minor commandment as a [seemingly] major commandment, because you do not know the value of each commandment. [Pirkei Avot 2:1]

Yes, there are hints that the 3 cardinal sins should be on top, or the Ten Commandments, or the 7 Noahide laws, or the "suprarational" commands (chukkim), or the commandments that recur most frequently, or that cost the most, or any commandments strongly emphasized by Chazal: "Idolatry is so serious a sin that its rejection fulfills the whole Torah. [Nedarim 25a]"; "Whoever speaks slander increases his sins even up to [the degree of] idolatry, sexual immorality and murder.[Arakhin 15b, Peah Y 1:1]"; "Tzedakah is as important as all the other commandments put together. [Bava Batra 9a]"; "Not giving tzedakah is tantamount to idolatry. [Ketubot 68a]"; "One mitzvah involving pain is worth a hundred mitzvot that do not." [Avot de Rabbi Natan 3:6, Jerusalem Talmud]; even "tzitzit is equal to all the mitzvot, because the gematria of tzitzit is 613!". When the Rambam put out his 13 primciples of faith, the reaction [Hasdai Crescas, Yosef Albo] was: Is the rest of Judaism any less important?

So there is no No. 1 mitzvah.

Answer 3

What is the ultimate sin?

Answer: The next one you are about to do now.

Why? Logic: The ones you have committed you cannot go back in time and avoid. The ones you have not done yet are not in your reality to deal with. So the ultimate sin is the one you can stop now. whatever it is.

As for the Cardinal three?

Traditionally it is because these are the three mentioned by which one must give up their life rather than commit them.

Other sins do not have the obligation of giving up one's life to avoid them.

See Talmud Sanhedrin 74a.

א"ר יוחנן משום ר"ש בן יהוצדק נימנו וגמרו בעליית בית נתזה בלוד כל עבירות שבתורה אם אומרין לאדם עבור ואל תהרג יעבור ואל יהרג חוץ מעבודת כוכבים וגילוי עריות ושפיכות דמים

§ The Gemara now considers which prohibitions are permitted in times of mortal danger. Rabbi Yoḥanan says in the name of Rabbi Shimon ben Yehotzadak: The Sages who discussed this issue counted the votes of those assembled and concluded in the upper story of the house of Nitza in the city of Lod: With regard to all other transgressions in the Torah, if a person is told: Transgress this prohibition and you will not be killed, he may transgress that prohibition and not be killed, because the preserving of his own life overrides all of the Torah’s prohibitions. This is the halakha concerning all prohibitions except for those of idol worship, forbidden sexual relations, and bloodshed. Concerning those prohibitions, one must allow himself to be killed rather than transgress them.

By the way, This Gemara from about this point (maybe some before too,) to the end of the chapter with Rashi and Tosfos is one of the most fundamental pieces of Judaism in the Talmud. Enjoy learning it. :)