There are at least 2 camps on this. You have stated the one fairly clearly.
I, and some other Messianics I know and know of, see things slightly different.
To us there are two branches of the Body of Messiah or the Fellowship of Believers, Christianity and Messianic Judaism.
The current situation is not unlike how things were in the first century, only reversed. Then, it was felt by many sincere Jewish believers that the Gentiles coming to faith in the Jewish Messiah naturally had to become Jews to be part of the Fellowship of Believers. As you know it was decided by the Counsel of Messiah in Jerusalem (Acts 15 & 21) that was not necessary. Thus there were (and remained until around 400CE) the two groups; "Jews who believe and are zealous for the Law" and "Gentiles which believe" who only had to keep the 4 Noachide laws.
After the draconian rulings of Constantine's counsel against Jewish believers the Messianics eventually melted away either into the Church or the Synagogue by around 400CE and the Hellenistic paradigm of culture, calendar and worship became the normal in Christianity for nearly 2000 years. The Body was like a one armed man and frequently lost it's balance through those millennia. It lost the steadying influence of Judaism.
Today, because this Christianity has the only known way of things for worship, festivals or of viewing scripture, the contemporary believer "knows" that all believers are and must be Christians and what all Christians do and believe in common, no matter their denomination. Any system outside of that which claims to follow Messiah is either a cult, or worse, a heresy.
However, Messianic Judaism does stand far outside of the recognized systems, views and calendar that is Christianity.
For many of us the Body of Messiah/Fellowship of Believers has both Christians - Jews and Gentiles who serve G-d through the institutions and traditions of the Church, and Messianics - Jews and Gentiles who serve
G-d through the institutions, traditions and Laws of Israel.
We are truly brothers (and sisters
) of the same Father and saved by the same Grace and stand as equals before His Throne. But just as men and women have different instructions or calling given by G-d, so too has the Jew and the Greek/Gentile.
The right hand can not say of the left hand "I don't need you." But neither can it say "I am a left hand."
For a more in depth treatment of these issues, and one more scholarly and well written, I offer the suggestion that you read my husband's book,
Jewish Roots: Reading the New Testament as a Jewish Document. It can be found on the same site as his
Galatians book.
b'Shalom {iPod touch w/CF app}