[NOTE: I am fully aware that what follows may be seen as off topic to this question, however I have enough experience with "messianic" groups and their beliefs to know that this is an absolutely essential qualification to make regarding this question.]
While the previous answer is sound indeed, many have tried to apply this reasoning to the texts associated with historical Jewish heretics, such as Jesus or the writings of the Qumran sectarians, whose manuscripts were either discovered or reinterpreted in relatively recent history.
To be clear, the above reasoning would apply only to those who were recognized as being Jews who genuinely possessed and transmitted authentic mesorah such as verifiable Geonim (a historically definable group; the heads of the Babylonian yeshivoth under Islamic rule) or a verifiable Rishon - as described above.
Jesus, however - although he has recently been re-interpreted by some as being a full-fledged "Pharisee" - is a Jewish heretic whose collected sayings (Jesus himself wrote nothing) cannot and will not ever be admitted to Jewish "canon". Not only because the "real" Jesus will always be shrouded in mystery due to the contradictory portrayals of his life, character, and teachings in the four "gospels" and other writings, but also because these writings contain many things that were innovations of Jesus and are contrary to the halakhah (both current halakhah and that of his own day), as well as false prophecies and a myriad of other issues. There is even an account in the Gospel of John where the Sanhedrin rejects him after openly performing melakhah on Shabbat (cf. John 9). It is unlikely that this event ever took place, however the fact that the author of John saw no issue in including it proves that he neither cared about nor sought the approval of the true leaders of Torah Judaism. These, and a great many other factors, preclude Jesus' teachings ever to be accepted into the body of authentic Jewish religious writings.
The Qumran sectarians bequeathed a vast amount of literature written in Hebrew, Aramaic, and Greek. The texts discovered there include religious calenders, legal rulings of their sect, mystical texts, liturgies, et al. Perhaps the most oft-quoted texts by those wanting to absorb their literature into the current Jewish "canon" are the Biblical manuscripts with variant readings from the Masoretic text. However, for similar reasons to Jesus, they are also disqualified. These groups were unambiguously sectarian; diverging from normative and authentic Judaism in manifold ways. They had strong Gnostic - and even dualist - tendencies, were aligned with the defunct and aberrant halakhic positions of the Sadducees, they followed a leader known as the "Teacher of Righteousness" whom they claimed was the sole mediator of salvation, and other things. They and those like them are fully and explicitly rejected by the sages of Israel in the records of the Mishnah, Tosefta, and the Talmudhim. Accordingly, in keeping with the halakhah recorded in Masekheth Shabbat 116a-b, their Torah scrolls are pasul and must be destroyed, even if written according to the halakhah.
There are also some equivocations that so-called "messianic" groups make when assessing either the Gospels, Qumran texts, or other pseudopigrapha in this manner:
- Jewish equals "Kosher" - This is the most common and widespread mistake in this regard. People feel that if they can prove that Jesus or the Qumran sectarians were Jews with religious ideas, then their writings/teachings are just as valid as any other Jewish literature. This is simply not correct. Jewish history is filled with a great number of heretics, most of whom held firm religious ideas, believed in God, and were convinced that their own ideas were correct. Nevertheless, they were rejected by the courts and authorized leaders of Israel and they remain heretics in perpetuity.
- Hebrew equals "Kosher" - This mistake is also extremely common. People feel that if something is written in Hebrew (or sometimes Aramaic), then it has a status and sanctity alongside the texts of Hazal or other authentic halakhic texts. For instance, many people try to redeem Jesus from being a heretic by translating the Greek texts of the "gospels" into Mishnaic Hebrew, as if the words of a heretic become acceptable if they contain rabbinic idiom. Rabbi Elisha` ben Abuyah (known also as "Aher") spoke in rabbinic idiom also, yet remains a heretic! Shabtai Tzvi - yimah shemo - also wrote in Hebrew, as did Theodore Herzl, yet they remain heretics in perpetuity. For this, and the reasons listed above, this idea is also false.
- Living in the Tannaitic era makes one a Tanna - I have read time and again that not only was Jesus a "Pharisee" but he was also a "Tanna" since he lived during the general era of the sages of the Mishnah. A "Tanna" means "one who communicates [authentic] halakhic teachings". Jesus did not do this, and had he done so, then his words would likely have been in the Mishnah or the Tosefta or one of the Baraytoth in the talmudhim. Neither Jesus nor the Qumran sectarians were "tannaim" simply because they were alive and expressed their respective religious ideas during that era.
- "Lo ba-shamayim hi" & "aharei rabbim le-hattoth" - These statements, found in Shemoth 23:2 and Devarim 30:12, mean "it [the Torah] is not in heaven" and "[one must] incline after the majority", respectively, and are foundational principles of halakhic authority. They absolutely preclude either Jesus, the "Teacher of Righteousness," or their respective sects, to innovate spiritual ideas or halakhic rulings outside the authority of the authorized Sanhedrin. Essentially, these two rules serve as important safeguards against charlatans and those claiming to have prophecy; those attempting to single-handedly corrupt or alter halakhic Judaism. Lo ba-shamayim hi means that miracles, fortune-telling, pseudo-prophecy, threats of divine retribution, etc. have absolutely NO standing in making halakhic decisions and are completely inadmissible in court (cf. Devarim 12-13). Thus, Jesus' and his followers' claims that his supposed "miracles" were somehow a proof of his message only shows their ignorance of true Torah law. Ignorance, or perhaps they were in open defiance of it. Aharei rabbim le-hattoth means that even if Jesus, or any other sectarian leader, were to sincerely hold to orthodox Judaism in every way while maintaining a different opinion of Jewish law such a person would be obligated to follow the majority ruling of the court. In fact, for someone to teach his own views as practical halakhah when they do not accord with the majority ruling(s) of the Sanhedrin carries with it a death penalty by strangulation (Mishneh Torah, Hilkhoth Mamrim 1-3).
What I have mentioned here is only a very small portion of what explains the eternal rejection of Jewish heretics and their writings. Much more could be written and elaborated upon, but this is not the place.
The point that I am trying to make clear is that the halakhic reasoning in the above answer can ONLY be applied to authentic and verifiable Jewish writings that might be discovered. It CANNOT be applied halakhically to sectarian and/or heretical texts, such as the "gospels" or those documents discovered at Qumran.