I'm not arguing that faith is not required and that the true Israel are those who were not only born in the lineage of Jacob but had faith in their Messiah. In Galatians the gentiles didn’t want to become physical Jews per se but were told they had to be circumcisied to be saved by the false teachers.
The gentiles wanted to become part of Abraham's family. That's why they got circumcised.
So don't worry, I'm not into a legalistic Judaism even though I believe God will be dealing with them directly in the near future.
The circumcision and the Law are not "legalism". They are the key symbols of the Jewish nationalistic pride. Saint Paul says that we should put aside "childish" things because we are now "men". He doesn't mean that we should despise the Law and and the Jewish religious exclusivism as they were some bad things. They are not bad things but preparatory means for the real circumcision (of the heart), for the real Abraham's children, for the real Law (of the New Covenant).
Paul says trust Christ alone, His work alone and do not put your faith in works [religious exclusivism] such as circumcision, Law etc. He talks about the Jews and their ethnical pride. And then He continues "is God only the God of the Jews?". No, His people are now all who believe in Christ.