Should Jewish Christians continue to observe the Torah? By which, I mean out of a spirit of thanksgiving, rather than works righteousness, much as the Apostle Paul says all Christians should strive to good works.
Do you mean the observance of circumcision, fringes, kosher, Sabbath, Passover, and ritual purity? What St. Paul wrote in Ephesians 2:12–15 makes me think even thankful Torah observance by Christian Jews would create an unnecessary distinction between Jews and Gentiles in a church. I also think those things are symbols of a covenant that came to an end and therefore any attachment to them is only nostalgic.
This is another instance of the pot calling the kettle black. Surely, one can believe that grace in Jesus alone is sufficient for salvation, but many will still note that the saints and Mary can increase the outpouring of grace. If that's so, then I can reasonably believe that works of the law (like circumcision, kosher, and sabbath observance) add to the deposit of grace throughout my Protestant walk in life. Paul simply meant that we now have grace sufficient enough for salvation in their absence, but that does not mean that observing them WON'T offer us MORE grace. I am currently authoring a new book for an evangelical megachurch called "The Man Centered Gospel", and it is meant to help you understand an emerging movement, quite ironically, titled, "The Man Centered Gospel". In it, we address the problem of "churches" being more about God and tedious liturgical practices than they are about more entertainment-focused formats such as Christian rock worship services, Stand-up Comedian pastors, and videos concerning end-times prophecy.
I certainly think so, when we add that latter proviso... Almost anything is acceptable when it is done in a spirit of charity and thanksgiving Also the Jewish identity has been miraculously preserved through the centuries, and I think it would be tragic for it to be lost when they agree to follow the Messiah... There's absolutely no reason for that!
I know some people accuse Jews for Jesus of promoting the Judaizer heresy, but my experience with messianics is that they are often more observant and Godly people than not only their Orthodox Jewish brethren, but even many evangelical Gentile Christians, and I find it quite inspiring. There is a verse in the bible that mentions something along the lines of a servant being freed from his obligations to his master, but the servant maintains that the master intended them for his wellbeing, and chooses to follow them anyway out of love in the wake of his newfound freedom. The destruction of the Jewish kingdom and priesthood and the subsequent abolishment of its Torah observances as an important mandate to be in good standing with God makes sense in its respective time frame. Orthodox Jews say that not all of the Old Testament observances are entirely worthless or out of date, and wonder why Paul makes so much fuss about doing away with them. But really, things like kosher required a kingdom observing Jewish law to encourage the proper foods to be cultivated for the Jews. The Jewish kingdom also respected observance of the sabbath day for those who could've instead been working. But since there was no longer a unique kingdom to enforce that, it lifted the burden of guilt for failing to uphold it, since the Jews would become scattered and be under the authority of Gentile rulers who would not respect their dietary laws (since they did not conform their eating habits to what is mentioned in scripture) or care if Jews employed as slaves should observe the Sabbath, which to their owners would be a hinderance. Again, we are not saved by those ordinances anymore (if we ever were), but most Messianics who still maintain them, more or less (as it varies among them) do not seem judgmental about the lack of observances in their fellow believing Jews or Gentile friends. It's just a matter of personal conscience. You will find very small, fringe heretical groups teaching the Judaizer heresy online but will likely never seem them in your average Messianic community. These groups have gentiles becoming "Jews" and their leaders outright denying the Trinity as a Gentile "heresy". Jews for Jesus, on the other hand, accepts many varieties of Jewish believers from all denominations, and even had an article about Messianic Catholics, including Anglican, Roman and Eastern Orthodox Jews. Anglicans were also very blessed to recieve the great scholar Alfred Eidersheim, who was a covert to the church from an Orthodox Jewish background and wrote some incredible books like The Life and Times of Jesus the Messiah.
The "Book of Witnesses" by David Kossoff is a really good read too. Indicative of a Jew who had a spirituual understanding of his Saviors teaching but never formally changed his "Jewish, love for God". And why ever should he, Paul never did. .