Very good question. Is orthodox Judaism the 'only' valid form/expression of Judaism? Without 'orthodox' Judaism Messianic Judaism would not exist? Orthodox Judaism is what God intended Judaism to be?
You see, as a Reform Jew from birth I have been pondering these questions from the beginning. But since 99.9% of the members here have not been raised within a form/expression of Judaism and have only 'discovered' your Jewish roots later in life, you seem to continually see Judaism as 'orthodox' only and have completely ignored the other branches of Judaism when it comes to learning your Jewish roots. Much to the counter missionary Jews delight. Because if they can get you to believe such a thing, then they have more power to convince you that your 'Messianic' form of Judaism is bogus. Leaving you with only one real option (in orthodox mentality), to convert to rabbinical Judaism. If you want to be a 'real' Jew.
It always floors me how Christians who enter into Messianic Judaism will identify with a form of Judaism that is based upon the rejection of Yeshua as an 'authentic' Judaism. It must be true, that if you say something long enough it becomes fact.
The apostles refered to gentiles following Jewish traditions as an unbearable yoke, and they were hunted down like animals by the teachers of the Law. Yeshua told them that they would be run out of their places of learning/worship, and the authorities would not confess Yeshua openly for fear of being put out of the synagogues. Even then it was against 'traditions' to claim faith in Messiah as a Jew. Messianics were NOT accepted by the 'pharisees', who after coordinating Messiah's death and fully rejecting the followers of Yeshua for not standing with them in defense against Rome formed 'Rabbincial' Judaism at Yavneh after the exile. Forming the antithesis of Messianic Judaism, which states YOU are the temple of God, by the power of the Spirit in living in you. Rabbinical Judaism was established upon a Judaism 'without' a temple, in complete rejection of what Messiah Yeshua has established, and accomplished. Namely the indwelling of the Spirit and the scattering of Israel into all the nations.
But hey, go ahead and follow them, if you will not listen to the Spirit of God within you. However, the name of Yeshua will not be spoken of in vain, without consequence. They have rejected the Holy One of Israel, the Lord of Hosts, the Savior of God. They reject the indwelling of the Spirit as given by Yeshua, God in the flesh who offered this gift to all who abide in him/them. WHAT do they know that YOU have not already been given? Namely, the Salvation of God!
Why are you going to them to learn the ways of Yeshua? Oh that's right, because you beleive Yeshua calls and leads you to practice (orthodox) Judaism......
The call is not be BE Jewish, it's to 'save' the Jewish. Through the Word of the gospel, and the indwelling of the Spirit offered. The call is for all Israel to be SAVED, and the nations will rejoice and be blessed. A non-Jew is to help the Jew become saved. Not become Jewish. ALL people need to be saved, but this comes through Israel finding our Messiah, as a nation. When Israel says 'blessed is he who comes in the name of Yeshua' then the nations will be blessed beyond measure. A non-Jews goal is to further this fact, support the Jews and bring them to the saving grace found ONLY in Yeshua and the indwelling of the Spirit.
Where is this taught in Rabbinical Judaism? This is ONLY taught in Messianic Judaism. Which was formed by and upon God himself, in the flesh, established with his own blood. And proved by the Spirit that lived within him, and by his life, death, and resurrection.
Yeshua is my authority, I have no other. He rules over his people, now and forever.