Jesus taught the disciples and others things they needed to know while he was here on earth. when he left the holy spirit with them , the holy spirit taught them everything, and all things. Paul was taught by God, and Jesus through the holy spirit,because he called him for that purpose. to preach to the gentiles first and then to the Jews.. so you see the teachings of Paul was not his own teaching but the teaching of Jesus Christ through the holy spirit. The reason they had so much trouble believing Paul's teaching was because they had not known that Jesus had called him to do so. they only knew him as the leader of those that was persecuting the church. it took some time before they trusted him.