why does Paul begin his letters by calling himself a called apostle?
Paul began his letters by calling himself a called apostle to establish the authority connected to his apostleship. In this age of grace, he was the authority through whom God spoke to the whole human race, so he defended that apostleship, especially against Jewish opposition that considered him a renegade.
Les explains that Paul's opening "called to be an apostle" is almost identical with the openings of all his other letters. 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 said they did not care what Paul said and would do what Jesus said—so Paul was up against the same challenge. He 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. Therefore he was always defending that apostleship, especially to the Jewish people, who thought he was a renegade to Judaism and had turned on his background.
Paul, called to be an apostle of Jesus Christ through the will of God, and Sosthenes our brother,