15 That tribulation then is profitable to the saints, that they may exercise moderation and lowliness, and that they may not be puffed up by their miracles and good works, and that God permits it for this end; we may hear David the prophet, and Paul saying the same. The former says, “It is good for me, Lord, that I have been in trouble, that I might learn your statutes:” and the latter having said, “I was caught up into the third heaven, and” transported to Paradise, goes on to say, “And lest I should be exalted above measure through the abundance of the revelations, there was given me a thorn in the flesh, the messenger of Satan to buffet me.” What can be clearer than this?
“That I might not be exalted above measure,” for this reason, says he, God permitted “the messengers of Satan to buffet me;” by messengers of Satan, indeed, he means not particular demons, but men ministering for the devil, the unbelievers, the tyrants, the heathens, who perseveringly molested, and unceasingly worried him. And what he says is just this: “God was able to repress these persecutions and successive tribulations; but since I had been caught up into the third heaven, and transported to Paradise, lest through the abundance of these revelations I might be lifted up and think much of myself, he permitted these persecutions, and suffered these messengers of Satan to buffet me with persecutions and afflictions, that I might not be too much exalted.”
For although Paul and Peter, and all that are like them, be holy and wonderful men, as indeed they are, yet they are but men, and require much caution lest they should be too easily exalted; and as saints more than others. For nothing is so apt to exalt to presumption as a conscience full of good works, and a soul that lives in confidence. To the end, therefore, that these might suffer nothing of this kind, God permitted that there should be temptations and tribulations; these being powerful to keep them down, and to persuade to the exercise of moderation in all things.
Source: Homilies on the Statues (New Advent)