From First Corinthians by George T. Montague, SM, commenting on I Corinthians 15:1-11
After adding his own testimony to the creedal recital, Paul is keenly aware of how his calling differed from that of the others mentioned. He had persecuted the church of God, and that should have made him unfit to be an apostle. But as the First Letter to Timothy will explain, “I was mercifully treated, so that in me, as the foremost, Christ Jesus might display all his patience as an example for those who would come to believe in him for everlasting life” (1 Tim 1:16).
God often calls the most unlikely, as he has done with the Corinthians (1 Cor 1:26–31). Not only did Paul not have good works of which he might have boasted (Phil 3:4–6); he also was the worst of sinners for having persecuted Jesus in his members (1 Tim 1:15). Hence he can say it is only by the grace of God that I am what I am. And that grace has continued to work in his life. He first says in a self-effacing way, His grace to me has not been ineffective, but then in a positive way he says he has toiled harder than all of them (the other apostles and evangelists).
He is not holding himself above the others, because whatever he has been able to do has been by the grace of God [that is] with me. After this brief expansion on his own ministry, Paul returns to the point made in 15:1—the one gospel that he handed on to them and they received. The apostolic witness to the resurrection of Jesus is not divided: whether it be I or they, so we preach and so you believed.