According to the haqdamah of the Mishneh Torah, it seems that any book(s) or commentary(s) which may have arisen after the hhatimath ha-talmudh ("the sealing of the Talmudh") - such as the writings of the geonim or even the Rambam's own book - are measured by their faithfulness to the halakhic and aggadic literature bequeathed to us by Hazal and their students (i.e. Mishnah, Tosefta, Mekhiltoth, Sifra, Sifre, Bavli, Yerushalmi, and the Tannaitic midrashim/baraytoth). Much like the prophets were tested against the collective mesorah up until their time, and by abrogation of it they themselves were rejected, so also are books composed since the close of the Talmudic age in need of such "testing."
Now, granted that there are two types of potential errors in such books:
- Unintentional errors of interpretation or logical understanding, etc. - In other words, a certain talmidh hakhamim codifies the halakhah as he sees it and explains hashqafah as he understands it from Tanakh and Hazal, but perhaps the halakhah is not like him and perhaps he misunderstands such issues as the nature of suffering or the function of sekhar wa-`onesh (or similar issues). These types of potential "mistakes" do not necessarily disqualify the author, rather we see that to attempt to understand Torah is a process where have to accept that different views of Torah from the sources is not only possible but probable.
- Departures from mesorah or attempts to replace it/abrogate it - Should a new book or treatise be written that stands in opposition to the halakhah or hashqafah as expressed by Tanakh and Hazal - especially that which seeks not to understand but to supplant - is to be rejected. Examples are "new revelations" that, rather than seek to understand the statements of Hazal in aggregate, attempt to make the case for "secret teachings" or "hidden meanings" that are in contradiction to established mesorah - such books and their authors are to be rejected.
[NOTE: I am aware that the above are fairly general and that it could be discussed in more detail, such as when to set aside midrashim in favor of peshat or outdated "scientific" ideas in order to incorporate new ones. However, for now these definitions should suffice for this answer.]
Certainly each of the above has limitations. For example, and perhaps most importantly, there are ideas about which different views are not acceptable and cannot be tolerated, such as the nature of the yihhudh HaShem, or the fact of a physical, bodily resurrection, or the permanence and immutability of the Torah. Such things (and those like them) define apiqorsim and minim (cf. Hilkhoth Teshuvah 3:14-17f) , and the Rambam - drawing on the text of the Mishnah and logical monotheism - composed his 13 Foundations of Jewish faith to show us where our speculation may go before it is undone and where we are not allowed to budge in our basic religious ideas and ideals.
Many books have come on the scene - both pre-Talmudic and post - claiming to be authentic to our mesorah, or to extend it, or to replace it. Examples include the "New Testament," the "Qur'aan," the "Kebra Nagast," the books of the Shabbateans (followers of Shabbetai Tzvi, yimahh shemo wa-zikhro, and many others. Many of them were accepted by great people. If Shelomoh HaMelekh could worship idols, if Elisha` ben Avuyah ("Ahher") could accept the idea of ribbui reshayoth from the books of the dualists, if Yohhanan Kohen Gadhol could become a Ssaduqi at the end of his life, if the Hakham Ssvi z"l could accept Shabbetai Tzvi (yimahh shemo) as the mashiyyahh, and if the Hafess Hhayyim z"l could be led to accept the blatantly forged (supposedly lost) Seder Qodhashin of the Talmudh Yerushalmi (and even changing his halakhic practices based on the forgery), then the fact that the Zohar was accepted by many great scholars when it first published should neither surprise us nor become the sure basis for its acceptance.
As an aside, one of the most common misconceptions is the equating of qabbalah with the Zohar literature; if the latter is rejected, it is thought, then the former ceases to exist. Such an idea is patently false, but nevertheless demonstrates how entrenched in the minds of contemporary Judaism is the idea that all authentic spirituality or "mysticism" in Judaism is inextricably linked to the ideas expressed in the Zohar. The fact is that the bodies of knowledge known as ma'aseh merqavah ("Workings of the Chariot" - i.e. metaphysics) and ma'aseh bereshith ("Workings of the Creation" - i.e. physics) - as mentioned in the Mishnah, masekhet Hhaghighah - preceded the 13th century publication of the Zohar by [possibly] thousands of years, as did the Sefer Yessirah. The Sefer Yessirah is referred to and expounded by the Kuzari and Sa'adyah Gaon, among others. The Rambam himself makes veiled references to these ideas in his Moreh HaNevokhim, expounding (where possible) mystical and philosophical concepts related to both ma'aseh bereshith and ma'aseh merqavah.
But this leads to another fact that is often overlooked in the history of the Zohar - many qabbalists at the time of its publication (and afterward) rejected it as being authentic. Rabbi Avraham Zacuto, in his Sefer HaYuhhasin, relates the extant portion of an account written by the well-known qabbalist Rabbi Yisshhaq De-Akko (a talmidh of the Ramban) who traveled to the home of Mosheh De Leon and offered to purchase the original manuscripts of the Zohar from his widow, whereupon she confessed to him that there were no original manuscripts and that her late husband had forged it and attributed it to Shimon ben Yohhai in an effort to gain acclaim and a higher purchase price. Other well-known qabbalists who rejected the Zohar as an authentic book of mesorah were Rabbi Ya'aqov Emden and the Hhathem Sofer (who was the student of the famed and intense mystic, Rabbi Nathan Adler). Their use of language is strong against the Zohar, using words like "forgery" and "lies" to describe it. All the while, however, these men and others maintained a highly-developed mystical system based on earlier literature.
The facts are clear to all who are willing to take an honest look: the Zohar contradicts a great many things which came before it in both the realm of halakhah and hashqafah - even surround such things as are "off limits" such as the nature of the yihhudh HaShem. And these things are well-known, they are not my invention nor the invention of secular scholars seeking to defame religion. They have been discussed and wrestled with for hundreds of years by rabbis and scholars in every area of Jewish literature. It has been proven that the Zohar borrows and incorporates sections of Rashi, Tosafoth, the Rambam, and other works which preceded it. It also contains a vast amount of original material, much of which is controversial when compared to works containing established mesorah from Hazal. This being the case, it is true that there are genuinely positive statements and spiritual truths expressed in the Zohar, however it is also true that there are severe statements of polytheism and dualism expressed there as well. So, the operating principle (it seems) is that anything good in the Zohar may already be found in uncontested and authentic works that preceded it, and anything questionable is of it's own invention. Such an observation makes the Zohar superfluous and the attempt to incorporate it into the corpus of Jewish literature as being more trouble that it is worth. The continuous heretical movements which base themselves upon it (e.g. the Shabbateans) and the seemingly endless stream of charlatans offering miracle cures, instant wealth, and superpowers of protection to those who embrace the Zoharic qabbalistic system are a proof that giving Zohar a prime place in Judaism has proven almost disastrous. The fact is that much of the good brought by hasidism could have been brought without the aid of Zoharic literature.
The historical Jewish response to the Zohar can be divided into three basic approaches:
- Full acceptance - The full acceptance of the Zohar and its attendant literature as being 100% authentic is most aptly characterized by the Hasidic movement(s) and the North African Sefaradim. These adherents hold it as the holiest text in Judaism and should be used to "correct" all other texts - especially those which came before it - which are viewed as being "ignorant" or "unaware" of the secret tradition that it holds.
- Modified acceptance - This approach, most commonly associated with the Gr"a and his talmidhim, is to effectively accept the Zohar, but to reject its commentaries. In other words, the Gr"a took great liberties to "re-read" the text of the Zohar in order to make it fit into the established mesorah. By doing so, he rejected many of the ideas of Lurianic qabbalah, and sought new readings (many of which are either based on his own emendations of the text or forced readings of the plain meaning of the Aramaic) to remove conflict and controversy. However, in doing so, the Gr"a also "re-reads" the text of the gemara in certain places and in some cases reverses generations of clear and uncontested pesaq halakhah from the gemara to accommodate the clear ruling of the Zohar to the contrary (one example of this is the wearing of tefillin on Hholo shel mo'edh).
- Full rejection - Characterized most aptly by the 19th-century Dor De'ah movement in Yemen led by Rav Yihhyah Qafihh z"l. Rav Qafihh authored a book entitled Milhhamoth HaShem ("The Wars of HaShem") wherein he effectively demonstrates (like other hakhamim before him) that the Zohar simply cannot be a product of Hazal and their students, is subsequently not an authentic work of mesorah, and therefore must be rejected. This and other groups rely instead on the works of previous and established authors for spirituality, such as the Rambam (Moreh HaNevokhim), Rabbi Yehudhah HaLewi (Kuzari), Rabbi Bahhyah ibn Pequdha (Hhovoth HaLevavoth), Sa'adyah Gaon (Pirush 'al Sefer HaYessirah and HaNivhhar Emunoth Wa-De'oth), and others.
We have a principle of "lo ba-shamayim hi", i.e. that the Torah is not "in heaven" and therefore we do not base our belief in any certain book or teacher based on purported "miracles" or claims of special "revelation" or "prophecy." Instead, we are charged with being faithful to the texts and the mesorah that we have to judge all that comes after it. This is why the latter two approaches (i.e. any approach beyond blind acceptance) take measures to study the relevant sources in order to formulate their opinions, rather than seeking a sign or relying on the fact that the likes of the Arizal gave it their approbation.
To answer your question directly, the Zohar has been accepted - and continues to be so - based on "mob rule" so to speak. In other words, since it has been read and used by a lot of Jews for a long time, we assume that it is true. In reality, however, there is no basis for its acceptance, but rather to the contrary. And as has been shown, there is nothing on the part of its supporters to substantiate their claims other than dogmas and the attribution of "special powers" or "revelations" or mystical "prophecy" on the part of those famed historical figures who did accept it, while attributing error and arrogance to those scholars who argued against it. It is exactly as you said, it is no different than the many false religious movements that have arisen in world history; they begin with charismatic and bold claims based essentially on nothing and demand blind obedience from all with whom they speak. But in the end, their claims are empty and their reasoning is circular.
Have you ever wondered why those who merely question the authenticity of the Zohar are threatened with excommunication and charges of heresy, while those who propose that a section of the gemara should be emended (and other such normal acts of Torah scholarship) are met with none of these? Le-`aniyuth Da'ati, it seems that those without truly substantive arguments have nothing left but threats of Divine judgment and ad hominem attacks. Sound familiar?
And PLEASE do not take my word for it - go and see for yourself. Investigate the matter thoroughly and with an open mind. If you come thereby to another conclusion, then you will have no threats and suffer no humiliation from me. And I certainly will not threaten you with a charge of "arrogance" for not seeing things the way that this or that scholar has.
I hope that this helps. Kol tuv.