Emerging almost entirely within a Protestant-Evangelical context, Messianic Judaism or Christianity (hereafter Messianic or Messianism), in its diversity of expressions, generally begins and operates with distinctively Christian Protestant assumptions and definitions. "One Law" or "One Torah" Messianics esteem the Torah, understood as the Pentateuch or the five books of Moses, as incumbent on Christians today. Using a post-Princeton, Protestant-Evangelical iteration of the doctrine sola scriptura Messianics operate, however artificially, with definitions and concepts inorganic to the very document which they esteem to be authoritative and so express a most basic form of anti-Semitism.
The Torah in Judaism is not just a document—it is not just the Pentateuch. For Jews of all sects and of all time, Torah is a process. The Torah as the Pentateuch is an authorized text and also an authorizing text—granting teaching and interpretive authority to the covenantal community who carries the torch from one generation to the next. To deny the covenantal continuity from the biblical era to the Jews of today is to deny the ethnic and pious experience of generations from the priest and the prophet to the scribe to the legal scholar to the rabbis. Such a denial is to deny the religiosity and piety of generations of Jewish people heralding from a diversity of historical-cultural locations yet within a unified and unfettered stream of transmission. Such a denial is itself the basest form of anti-Semitism.
As already indicated, Torah is more than Pentateuch. Deuteronomy 17:8ff, in commissioning the existence of a centralized authority structure, presents the priests and judges as authorized agents of interpretation. Regarding their verdicts in matters of legality and tort, Deuteronomy 17:11 states,
According to the torah (הַתּוֹרָה) which they shall teach thee, and according to the judgment which they shall tell thee, thou shalt do; thou shalt not turn aside from the sentence (הַמִּשְׁפָּט) which they shall declare unto thee, to the right hand, nor to the left.
The didactic role of the priests (cf Deut 33:10) and the judges is authorized here. The people are to learn torah/instruction from within the covenantal context—not alone in a supposed vacuum of private interpretation. Malachi 2:7 expresses a similar legal priority:
For the priest's lips should keep knowledge, and they should seek the law at his mouth; for he is the messenger of the LORD of hosts.
Torah/instruction is not to be sought from a book, from the Pentateuch but from a person. This chain of command, if you will, ensures the integrity of covenantal continuity and the passing of the torch not of a dead letter but of a living tradition. Messianics today attempt to keep Torah as a document. They import concepts of scriptural authority onto the text, concepts which deny the experience of the Jewish people and attempt to artificially construct the Pentateuch into a document that can stand alone, without authoritative interpretation. As a result of their confused amalgam between the Pentateuch and Protestant piety, the end result is an insult to Judaism and an impossible matrix of observance with such fundamental differences of observance as to preclude enduring community. Yet, frankly, to accept the authenticity of Jewish religiosity from the biblical era to today would necessarily negate their identity today.
Much has not been said here. I have left a lot undefined and undefended, and I hope that more will be fleshed out in any replies or in further posts. I group this topically in post-modernism as it expresses the importance of historical-cultural location awareness.