2 Corinthians 12:7-9 (NIV)
To keep me from becoming conceited because of these surpassingly great revelations, there was given me a thorn in my flesh, a messenger of Satan, to torment me. Three times I pleaded with the Lord to take it away from me. But he said to me, "My grace is sufficient for you, for my power is made perfect in weakness." Therefore I will boast all the more gladly about my weaknesses, so that Christ's power may rest on me.
God used Paul in a mighty way, perhaps more so than any other person we know of in the Bible. Paul spoke of himself in the third person earlier in this chapter how he "was caught up to paradise. He heard inexpressible things, things that man is not permitted to tell." What mystery this presents to us! Paul was different from all the other apostles. He was discipled personally by Jesus like the others but not until after Jesus' resurrection and ascension. No doubt this revelation was part of that instruction. The interesting thing about this revelation is that Paul was not allowed to tell it like so much other revealed doctrine which he wrote down for us in his letters.
This intriguing and tantalizing reference was made solely for the purpose of explaining the thorn in the flesh Paul was given. This thorn was a reminder to Paul and kept him humble and dependant on the Lord. Here again we have God allowing suffering not for punishment but for a loving purpose, in this case to make Paul better able to do God's work. We don't know what the thorn was; some speculate it was some sort of eye disease.
Here is the wonderful principle for us: if we do something great for God with our natural ability, then people might give us the praise. But if we do something supernatural for God out of our weakness, the praise must go to Him since it is unexplained apart from His power.