By the Grace of God
1 Corinthians 15:10 (ESV) … “But by the grace of God I am what I am, and his grace toward me was not in vain. On the contrary, I worked harder than any of them, though it was not I, but the grace of God that is with me.”
Neither Paul nor any of the apostles could take credit for achieving the position of apostle. They had all been called to that position by the grace of God. Only by God’s “grace”—his undeserved favor poured out on sinners—was Paul saved and enabled to serve. And Paul certainly did so! He wrote of having worked harder than all of the other apostles.
This was not an arrogant boast because he knew that his hard work was a result of the grace of God that was with him. Because of his previous position as a Pharisee (Acts 23:6; Philippians 3:5) and his previous occupation of persecuting Christians, Paul’s conversion made him the object of even greater persecution than the other apostles; thus he had to work harder to preach the same message. He wrote to Timothy, “The saying is sure and worthy of full acceptance, that Christ Jesus came into the world to save sinners—of whom I am the foremost. But for that very reason I received mercy, so that in me, as the foremost, Jesus Christ might display the utmost patience, making me an example to those who would come to believe in him for eternal life” (1 Timothy 1:15–16 nrsv).
 Barton, B. B., & Osborne, G. R. (1999). 1 & 2 Corinthians (pp. 222–223). Tyndale House.