From: 2 Corinthians 12:7-10
Visions and Revelations
 And to keep me [Paul] from being too elated by the abundance of revelations, a thorn was given me in the flesh, a messenger of Satan, to harass me, to keep me from being too elated.  Three times I besought the Lord about this, that it should leave me;  but he said to me, "My grace is sufficient for you, for my power is made perfect in weakness." I will all the more gladly boast of my weaknesses, that the power of Christ may rest upon me.  For the sake of Christ, then, I am content with weaknesses, insults, hard- ships, persecutions, and calamities; for when I am weak, then I am strong.
7-10. Displaying admirable humility, St Paul now refers to the weakness God allowed him to experience to ensure his supernatural gifts did not make him proud. It is impossible to say what exactly the "thorn in his flesh" was. Some Fathers- St Augustine, for example--and modern commentators think that it was some particularly painful and humiliating physical ailment, possibly the same one as he refers to in Galatians 4:13f, where he also speaks in general terms. Others, like St John Chrysostom, are of the view that he is referring to the pain which continual persecution caused him. Others--from St Gregory the Great onwards --opt for an ascetical interpretation; they say he is referring to temptations to do with conscience; but the supporters of the two other theses argue, for example, that it is unlikely that St Paul would have mentioned anything of that kind, because it could have given his enemies ammunition for further attacks.
St Paul asked God to take this "thorn" away, but the heavenly answer he received is very revealing: God's grace is enough to enable him to cope with this difficulty--which serves to reveal God's power. And so it is that he boasts of and is content with his weaknesses and the persecution he suffers: in these circumstances he is stronger than ever, thanks to God's supernatural help.
When commenting on this passage, St Thomas explains that God sometimes permits certain kinds of evil in order to draw out greater good: for example, in order to protect people from pride--the root of all vices -- he sometimes allows his chosen ones to be humiliated by an illness, or a defect, or even by mortal sin, in order that "the person who is humbled in this way might recognize that he cannot stand firm by his own efforts alone. Hence it is said in Romans 8:28, 'We know that in everything God works for good with those who love him'--not of course that God seeks the sin but [the sinner's] turning to him" ("Commentary on 2 Cor, ad loc.").
7. "A messenger of Satan", an angel of Satan: this is how he describes the humiliating "thorn". This suggests that the disability could have been seen as an obstacle to his work of evangelization--which the devil, logically, would have been keen to frustrate (cf. 2:11; 11:14f).
8-10. Christians can learn a great deal about the ascetical struggle from these words. They remind us, on the one hand, of the need to ask the Lord to help us when we experience difficulties, and at the same time to be full of trust and to abandon ourselves to God, who knows what is best for us. "The Lord is good", St Jerome teaches, "because he often does not give us what we desire, in order to give us something we would prefer" ("Epist. ad Paulinum").
The passage also shows us what attitude we should take to our own weakness: "We have to glory", St Alphonsus says, "in the knowledge of our own weakness in order to acquire the strength of Jesus Christ, which is holy humility", without "giving in to lack of confidence, as the devil wants, and falling into more serious sins" ("Treasury of Preaching Material", II, 6).
At the same time this passage teaches us that awareness of our personal shortcomings should lead us to put all our trust in God: "We have to cry out ceaselessly with a strong and humble faith, 'Lord, put not your trust in me. But I, I put my trust in you. ' Then, as we sense in our hearts the love, the compassion, the tenderness of Christ's gaze upon us (for he never abandons us) we shall come to understand the full meaning of those words of St Paul, "virtus in infirmitate perficitur" (2 Cor 12:9). If we have faith in our Lord, in spite of our failings -- or rather, with our failings--we shall be faithful to our Father, God; his divine power will shine forth in us, sustaining us in our weakness" (St. J. Escriva, "Friends of God", 194).
Source: "The Navarre Bible: Text and Commentaries". Biblical text taken from the Revised Standard Version and New Vulgate. Commentaries made by members of the Faculty of Theology of the University of Navarre, Spain. Published by Four Courts Press, Kill Lane, Blackrock, Co. Dublin, Ireland. Reprinted with permission from Four Courts Press and Scepter Publishers, the U.S. publisher.