Why did Paul have to defend his apostleship especially to Jewish people?
Paul had to defend his apostleship to Jewish people because they regarded him as a renegade to Judaism who had turned on his background. He also needed to establish that, for believers in the age of grace, he was the authority through whom God was speaking to the whole human race.
Les explains that Paul’s opening in the epistle is almost identical to his other letters, and the first thing Paul had to establish to whomever he wrote was the authority connected to his apostleship. The Corinthians had divisions: some claimed to follow Paul, others Peter, and others dismissed Paul, saying they would do what Jesus said. Paul had to establish that for believers in this age of grace, he is the authority through whom God is speaking to the whole human race. He was always defending that apostleship, especially to the Jewish people, because they thought he was a renegade to Judaism and had turned on his background.