Many, not all. Paul’s letters established the practice of converting to Christianity directly from polytheism without assuming Jewish practices first.

That said, on the eve of Christianity, Many Romans casually adopted parts of Jewish belief and practice, sort of like celebrities in the 2000s doing Kabbalah. So the line of who was and wasn’t Jewish was kinda blurry. At least before the council of Nicea and Constantine, most Christians were probably considered Jews by the Romans, and Heretics by the nascent Rabbinic movement.