For I am the least of the apostles, that am not meet to be called an apostle, because I persecuted the church of God.

People's New Testament

15:9 For I am the least of the apostles. As far as human worth is concerned, not fit to be called an apostle.

Because I persecuted the church of God. He could never forget that he had been a persecutor. See Ac 9:1,2 22:04 26:11:00 Ga 1:13 Php 3:6 1Ti 1:13.

Wesley's Notes

15:9 I persecuted the church - True believers are humbled all their lives, even for the sins they committed before they believed.

Jamieson-Fausset-Brown Bible Commentary

9. least-The name, "Paulus," in Latin, means "least."

I persecuted the church-Though God has forgiven him, Paul can hardly forgive himself at the remembrance of his past sin.

Matthew Henry's Concise Commentary

15:1-11 The word resurrection, usually points out our existence beyond the grave. Of the apostle's doctrine not a trace can be found in all the teaching of philosophers. The doctrine of Christ's death and resurrection, is the foundation of Christianity. Remove this, and all our hopes for eternity sink at once. And it is by holding this truth firm, that Christians stand in the day of trial, and are kept faithful to God. We believe in vain, unless we keep in the faith of the gospel. This truth is confirmed by Old Testament prophecies; and many saw Christ after he was risen. This apostle was highly favoured, but he always had a low opinion of himself, and expressed it. When sinners are, by Divine grace, turned into saints, God causes the remembrance of former sins to make them humble, diligent, and faithful. He ascribes to Divine grace all that was valuable in him. True believers, though not ignorant of what the Lord has done for, in, and by them, yet when they look at their whole conduct and their obligations, they are led to feel that none are so worthless as they are. All true Christians believe that Jesus Christ, and him crucified, and then risen from the dead, is the sun and substance of Christianity. All the apostles agreed in this testimony; by this faith they lived, and in this faith they died.