2 Cor. 12:5 Of such a one I will boast; yet of myself I will not boast, except in my infirmities.
Paul was more than qualified to brag in his accomplishments. However, Paul also knew those accomplishments came from the LORD. Those truly belong to God, for His glory. But what Paul, and all humanity has, is weaknesses. When our weaknesses are handed over to God, the LORD replaces them for strength. We know the results of what God can do in us. We can testify of those changes.