I think it is even more basic than simply Hebrew Jews and Helenist Jews. When the Babylonian Captivity ended, the majority of Jews remained in Babylon, and Babylon continued to be the seat of Judaism for a few hundred years. A smaller number of Jews dispersed throughout the world, and the smallest number of Jews returned to Judea, backslid from the goal of Judaism(resurrection of the kingdom), and became convinced that they were the only true Jews, and Jerusalem was the seat of Judaism. They were so sure of this that they killed all the prophets sent from Babylon to get Jerusalem Judaism back to the true Jewish goal. Judean Jews considered any Jews who did not recognize Jerusalem as the seat of Judaism, and Jerusalem Judaism as the only true form of Judaism, to be gentiles.
Paul was obviously following the gospel formula by converting backslidden Jews, and gentiles, into "True Jews/True Christians".
"I don't speak for God. All I have is personal opinion based on the story of continuity which runs through the bible!"