There’s only one gospel, but where is that gospel from? Is it just an invention, an eccentric interpretation from a very eccentric guy? Paul defended his apostleship, and now he defends his message. The gospel is objectively true whether Paul believes it or not, but because he believes, it has changed his life. God’s grace not only changes the present, it teaches him that his whole life has been the plan of a sovereign God who is using it for His glory.