What was the disagreement between the Jerusalem believers and the Gentile converts in Antioch?
The disagreement was whether Gentile converts must be circumcised and keep the Law to be saved. Jerusalem believers, including believing Pharisees, demanded circumcision and Law observance, saying faith alone was not enough. The Gentile converts in Antioch, having received Paul's Gospel, stood on salvation by faith alone.
Les explains that the bone of contention was between the Jerusalem Jewish believers and the Gentile converts in Antioch. The believing Pharisees, who had embraced Jesus as Messiah, said it was needful to circumcise the Gentiles and command them to keep the Law. They taught that circumcision plus Law-keeping was necessary, or the Gentiles could not be saved. The Gentile converts had received Paul's Gospel, which was based on faith alone, so the dispute was whether faith alone was sufficient or whether the Law had to be added. Les notes this same demand persists today: people say they believe the Gospel but claim it is not enough alone.