According to God's Word, John the Baptist and Lord Jesus himself had affirmed that revelation long before Paul. Judaism before Christ was a religion, not a monolithic ethic group, anyone converted into Judaism was a Jew.Those revelations that Paul received were not known when, through the Jews, the church was born. A Jew, when the church was born, were those born of the twelve tribes. The first mention of a proselyte Jew is in Acts 6:5, Nicolas of Antioch. There is no other mention of proselytes earlier, so we can't assume there were, even from the surrounding regions. If you do that's just your opinion, not God's Word.
Do not think to say to yourselves, ‘We have Abraham as our father.’ For I say to you that God is able to raise up children to Abraham from these stones. (Matt. 3:9)
For whoever does the will of My Father in heaven is My brother and sister and mother. (Matt. 12:50)
Woe to you, scribes and Pharisees, hypocrites! For you travel land and sea to win one proselyte, and when he is won, you make him twice as much a son of hell as yourselves. (Matt. 23:15)
Also, ever since the Exodus, there was a "mixed multitude" following the Israelites into the wilderness, Ex. 12:37-38, they accepted the Torah as well as the descendants of the twelve tribes; and since Rehoboam, Israel split into the Northern and the Southern kingdoms, the northerners were conquered by the Assyrians and scattered abroad, known as the Lost Ten Tribes, only the descendants of Judah and Benjamin in the south counted as Jews; by the time of Jesus, Jews who settled in the former northern kingdom and intermarried with other peoples were ostracized as Samaritans, they were no longer Jews as well. Therefore, "a Jew, when the church was born, were those born of the twelve tribes" is absolutely false.