7 See Paul a small portion of this inheritance, see him in weakness, who said, “I am not meet to be called an Apostle, because I persecuted the Church of God.” Why then are you an Apostle? “By the grace of God I am what I am. I am not meet, but by the grace of God I am what I am.” Paul was “weak,” but You have “perfected” him. But now because by “the grace of God he is what he is,” look what follows; “And His grace in me was not in vain, but I laboured more abundantly than they all.” Take heed lest you lose by presumption what you have attained through weakness.
This is well, very well; that “I am not meet to be called an Apostle. By His grace I am what I am, and His grace in me was not in vain:” all most excellent. But, “I laboured more abundantly than they all;” you have begun, it would seem, to ascribe to yourself what a little before you had given to God. Attend and follow on; “Yet not I, but the grace of God with me.” Well! You weak one; you shall be exalted in exceeding strength, seeing you are not unthankful. You are the very same Paul, little in yourself; and great in the Lord.
You are he who thrice beseeched the Lord, that “the thorn of the flesh, the messenger of Satan, by whom you were buffeted, might be taken away from you.” And what was said to you? What did you hear when you made this petition? “My grace is sufficient for you: for My strength is made perfect in weakness.” For he was “weak,” but Thou “perfected” him.
Source: Sermons on Selected Lessons of the New Testament (New Advent)