Paul s teachings do not contradict the Old Covenant but comply with it. Understanding this will cause much of what he wrote to become clearer to all students of the Bible whether Jewish or non-Jewish. As touching the Gentiles which believe, we have written and concluded that they observe no such thing, save only that they keep themselves from things offered to idols, and from blood, and from strangled, and from fornication . He did teach the Gentiles according to Rabbinic law pertaining to the Gentiles, which has it s origins in the Old Covenant. This also will any Rabbi do today, should you care to check it out.
Gentiles can never become Jewish, ethnically speaking.
Gentiles can become a part of the Abrahamic and or Mosaic covenants, but being Jewish is a matter of birth. It is the same difference as being born Chinese or Indian. Gentiles can embrace Jewish teachings, convert to the Jewish faith and become a teacher of Jewish ways, but they can never become ethnically Jewish. They can have a circumcised heart and even appear to be more Jewish than many ethnic Jews, but to be a Jew one must be a descendant of Jacob. Even though grafted in, they maintain their uniqueness while remaining part of the good tree.