Jamieson, Fausset and Brown on 1 Corinthians 15:10

10. by ... grace ... and his grace--The repetition implies the prominence which God's grace had in his mind, as the sole cause of his marvellous conversion and subsequent labors. Though "not meet to be called an apostle," grace has given him, in Christ, the meetness needed for the office. Translate as the Greek, "His grace which was (showed) towards me."

what I am--occupying the honorable office of an apostle. Contrast with this the self-sufficient prayer of another Pharisee (Lu 18:11).

but I laboured--by God's grace (Php 2:16).

than they all--than any of the apostles (1Co 15:7).

grace of God ... with me--Compare "the Lord working with them" (Mr 16:20). The oldest manuscripts omit "which was." The "not I, but grace," implies, that though the human will concurred with God when brought by His Spirit into conformity with His will, yet "grace" so preponderated in the work, that his own co-operation is regarded as nothing, and grace as virtually the sole agent. (Compare 1Co 3:9; Mt 10:20; 2Co 6:1; Php 2:12, 13).