One of the basic principles of Judaism is that when God gave the written text of the Torah to Moses at Sinai, He also gave over to Moses an Oral Torah. This Oral Torah is the authoritative interpretation of the written Torah, telling us how to perform the various commandments, and also providing us with a methodology for Scriptural interpretation (hermeneutic rules). Today, the teachings of the Oral Torah are embodied in the Talmudic literature (which includes much more than just the Talmud itself).
People often ask what justification is there for the belief in the Oral Torah. This question is sometimes rooted in a discomfort with the idea of giving human beings dispositive authority over the Torah. For these questioners, while they are willing to accept the authority of the written Torah as a Divinely revealed document, they are uncomfortable with giving authority to human beings to determine what the Torah really means.
More often, however, the question is motivated by a desire to give legitimacy to alternative interpretations. If, the argument effectively goes, there is no Divinely revealed Oral Torah, then my interpretation (or my sect/denomination/religion’s interpretation) has as much authority as any other. For some, such as the liberal Jewish movements, even the written Torah has little inherent authority, and the denial of the Oral Torah is just part of a larger argument against traditional authority. For others, such as for many Christian believers, the Divine origin of the written text of the Torah is fully acknowledged, but their understanding of the Scriptures differs dramatically from the traditional Jewish interpretations. They are therefore forced to challenge the claim that the traditional Jewish interpretation, i.e. the Oral Torah, is of Divine origin.
(In most cases, people who believe themselves to be in the first group are actually in the second. Their difficulty is that the preconceptions underlying their interpretation of Scripture are so taken for granted that they do not realize that they are engaged in interpretation in the first place.)
For the first group, the basic answer is that, while their concern is understandable, the fact remains that God wrote the written Torah in such a way that there is simply no way to avoid the need for human interpretation. Any attempt to use Scripture as a practical authority requires interpretation of the Scriptures and all such interpretation involves preconceptions that exist, at least in part, independently of the text. This is true even for those groups that entirely deny the existence of such a tradition. The Sadducees, the Karaites, the Christians, and even the modern “liberal” Jewish denominations (e.g. Reform) all have their own approaches to Scriptural interpretation that conform to and support their various beliefs and ideologies.
The point being that you can't avoid utilizing some kind of interpretive preconceptions in studying Scripture. A true "sola scriptura" approach has never really been possible. For someone who accepts the Divine origin of the Torah, there is no way of avoiding the fact that God clearly wrote the Torah to be interpreted by human authorities. This is actually explicitly stated in Deuteronomy 17:11, “According to the Torah which they shall teach you, and according to the judgment which they shall tell you, you shall do; do not not turn aside from what they shall tell you, to the right or to the left.”
The existence of an unwritten Divinely revealed interpretive tradition is implicit in many places in the Torah. The mere fact that numerous major obligations (e.g. tefillin, mezuzah, and the prohibition against “work” on the Sabbath) are imposed without any clarity as to their practical nature indicates that this is so. The prohibition against work on the Sabbath is particularly telling, as Scripture mandates the death penalty for its violation. Is it really plausible that God would instruct us to give the death penalty for a crime which is never clearly defined? I mean, what, exactly, counts as "work"? Is this really something that we are supposed to determine on an ad hoc basis?
There are also explicit Scriptural references to Divinely revealed teachings that are not to be found in the Written Torah (e.g. Deut. 12:21 re:the laws of kosher slaughter).
These facts clearly indicate that the existence of an authoritative oral tradition is not only supported by Scripture, but is actually demanded by it.
Obviously, those who deny, to whatever degree, the Divine origin of the Torah, will similarly have no reason to accept the authority of the Oral Torah. But for those who accept the authority of the Torah as a Divinely revealed document, there is no way to avoid the necessity of accepting that there is also an authoritative interpretive tradition that was revealed by God together with the Torah.
The question that remains is to determine which interpretive tradition has the strongest claim to authenticity. The first thing to determine is, from a historical point of view, whether a given tradition can plausibly claim to have originated at Sinai together with the Written Torah. Obviously, interpretive traditions that first came into existence long after the Sinai Revelation simply don’t qualify.
Once we have eliminated all such obvious latecomers, we then have to deal with the second question. This is whether the specific interpretive tradition can be said to be consistent with the basic themes of the Written Torah. While the premise of the Oral Torah gives the Oral Torah the authority to interpret the Written Torah, this does not mean that the Written Torah is simply an inert piece of clay that can be reshaped at will. The Torah may be vague on some points, and there is certainly plenty of room for non-literal interpretation of many verses, but there are certain themes that are so central to the Jewish Scriptures that there is simply no way to deny them. An interpretive tradition that clearly flies in the face of these central themes is not plausibly authentic.
Of the existing, living, interpretive traditions, there are really only two that can can plausibly claim to be of genuine antiquity, the Rabbinic/Talmudic tradition and the Christian tradition. (I am using the term “Christian” in the broadest possible sense, referring to the mainstream beliefs held in common by most of the innumerable sects, denominations, and religions that fall under that term.)
Christianity claims to be fully in consonance with the teachings of the Jewish Scriptures, which Christianity refers to as the “Old Testament.” Indeed, Christianity claims that the Old Testament clearly attests to the truth of Christianity, and that the Christian interpretation of the Jewish Scriptures to this effect is not an innovation, but reflects the true interpretation and that the denial of this interpretation by the overwhelming majority of Jews who lived in the time of Jesus was a falsification of the true, indeed, self-evident, testimony of the Hebrew Scriptures.
The problem with this claim is that it is simply not supported by the facts. On the contrary, many of the basic concepts of Christianity - e.g. the identification of the human messiah with God (“the divinity of Christ”), the idea of the messiah coming (and then dying) without bringing about any significant political change, the abrogation of the laws of the Torah, and the supersession of the Jewish people by the Christian church - are fundamentally inconsistent with what one would get from a straight reading of the Hebrew Scriptures. These inconsistencies are not on minor details but on major Scriptural themes (e.g. idolatry, the eternal nature of the laws, the role and destiny of the Jewish people, the function of the messiah).
Accepting the basic premises of Christianity therefore has the effect not only of rendering the “Old Testament” a “dead letter”, but an extremely misleading and illogical “dead letter”. The most basic themes of the Hebrew Scripture are disposed of entirely, and are replaced with ideas that are completely unsupported by the text. It is simply not plausible that this was the authoritative interpretation given to the Jewish people at Sinai. (This is asides from the simple matter that, if it had been, then the Jewish people wouldn’t have rejected Jesus in the first place. It is this inconvenient fact that forced medieval Christians to conclude that the Jews were a consciously evil people, who knowingly denied the divinity of Jesus. From there it was only a short jump to the insane accusations of “Host desecration” and the blood libel that resulted in the death of untold thousands of innocent Jews.)
I am not going to attempt here to respond to the innumerable so-called “proof-texts” cited by Christians to support their claims (except to say that none of these “proofs” hold up under serious analysis). The question of how to deal with specific verses is not my topic here. There are all kinds of interesting verses in Scripture that can be interpreted in any number of ways, and misinterpreted in even more. The question is the underlying assumptions involved in that interpretation. If the underlying assumptions are wrong, then it is inevitable that the interpretation will be wrong as well.
The point in this post is not to critique Christianity. The point is to establish that the “Oral Torah”, as reflected in the Rabbinic tradition, is the only plausible candidate for an interpretive tradition of Divine origin, and that the need for such an interpretation is both self-evident and clearly attested to in Scripture. In practical terms, therefore, despite whatever questions one might have about the Rabbinic tradition itself, it remains the only viable candidate.
I would stress that, while here I am only making the case for the authority of the Rabbinic tradition by a “process of elimination”, in that there simply are no other plausible candidates, the case for the Oral Law is actually far stronger than that. There certainly are more direct arguments for the validity of the Rabbinic tradition. However, these arguments require studying many of the details of the Rabbinic tradition and would be too lengthy and technical for me to attempt in this post (and would also require a degree of competence in Talmudic study on the part of the reader).