To Paul was committed the greatest revelation of all time. He was divinely commissioned to proclaim the glorious all-sufficiency of Christ’s redemptive work, God’s offer of salvation by free grace to all who trust in Christ and their heavenly position, blessings and prospect.
Lest he should become puffed up by the glory of these great truths, God gave him what he calls “a thorn in the flesh”, an aggravating physical infirmity of some sort. “For this thing,” he says, “I besought the Lord thrice [three times], that it might depart from me” (II Cor. 12:8). But the Lord knew better than Paul what was best for him:
“And He said unto me, My grace is sufficient for thee; for My strength is made perfect in weakness” (II Cor. 12:9).
How right God was! Every Christian knows that with brimming health and “good fortune” comes the tendency to forget our need of Him, while infirmity causes us to lean harder and to pray more and this is where our spiritual power lies. Every believer should acknowledge this and say with Paul:
“Most gladly therefore will I rather glory in my infirmities, that the power of Christ may rest upon me. Therefore I take pleasure in infirmities…for when I am weak, then am I strong” (II Cor. 12:9,10).
Infirmities of the flesh are common even to God’s choicest saints. What satisfaction there is, then, in just believing God’s Word: “My grace is sufficient for thee, for My strength is made perfect in weakness”. source