Paul sometimes had to defend his ministry, but he was often self-deprecating, exercising humility. Humility isn't acting less important than someone else, it is realizing what you actually are.

1 Corinthians 1:22   For the Jews require a sign, and the Greeks seek after wisdom:
1 Corinthians 1:23   But we preach Christ crucified, unto the Jews a stumblingblock, and unto the Greeks foolishness;
1 Corinthians 1:24   But unto them which are called, both Jews and Greeks, Christ the power of God, and the wisdom of God.
1 Corinthians 1:25   Because the foolishness of God is wiser than men; and the weakness of God is stronger than men.
1 Corinthians 1:26   For ye see your calling, brethren, how that not many wise men after the flesh, not many mighty, not many noble, are called:
1 Corinthians 1:27   But God hath chosen the foolish things of the world to confound the wise; and God hath chosen the weak things of the world to confound the things which are mighty;

1 Corinthians 1:28   And base things of the world, and things which are despised, hath God chosen, yea, and things which are not, to bring to nought things that are:
1 Corinthians 1:29   That no flesh should glory in his presence.

God has used us despite of ourselves.