1 Corinthians 15:8-10 And last of all he was seen of me also, as of one born out of due time. For I am the least of the apostles, that am not meet to be called an apostle, because I persecuted the church of God. But by the grace of God I am what I am: and his grace which was bestowed upon me was not in vain; but I laboured more abundantly than they all: yet not I, but the grace of God which was with me.
In Paul's own words we hear him claiming to have both persecuted the church, and to have worked more than all of the other apostles. That last claim couldn't have landed well on Peter's ears, but none of the apostles deny this boast. In fact, Peter has great things to say about Paul's letters, calling him wise in God, learned, and sometimes hard for the average man to understand.
2 Peter 3:15-16 ... even as our beloved brother Paul also according to the wisdom given unto him hath written unto you; As also in all his epistles, speaking in them of these things; in which are some things hard to be understood, which they that are unlearned and unstable wrest, as they do also the other scriptures, unto their own destruction.
Paul concludes the summary of Jesus' appearances after the cross with the simple defense, "I am what I am, by the grace of God." It didn't matter how many he persecuted. It didn't matter how late he came into the ministry of Jesus. It didn't matter if his meeting with Jesus came before Calvary or after...he was what he was.
And so are you and I. Just another case of God's little "I AM" applying to His children.