Therefore in order that I might not be puffed up, there was given to me a thorn in the flesh, an angel of Satan, in order that he might torment me, in order that I might not be puffed up.
It's quite irrelevant what the thorn actually was (physical infirmity, persecution, temptation, etc). I wonder if Paul was deliberately ambiguous so that we wouldn't be distracted and miss his real point: not the content of the thorn, but the intent. He says it twice (some translations only have it once): "in order that I might not be puffed up."
On behalf of this three times I exhorted the Lord in order that he might take it away from me. And he has said to me, "Sufficient for you is my grace, for my power is completed in weakness. All the more gladly, therefore, will I boast in my weaknesses [general heading, followed by increasingly difficult experiences?] in insults, in hardships, in persecutions, in anguishes, on Christ's behalf; for whenever I am weak, then I am strong.
I'm prompted to post this passage just because every time I read it (unless I'm too sleepy and not concentrating) I find it extremely deconstructive of my heart. Strength is found not by avoiding, overcoming, or denying weakness, but in it. It is a truth that is so counterintuitive. You think you've grasped it only to find a week later you're right back living out of strength, frustrated at the weaknesses in your life which seem to be keeping you from your goals. And Paul says, "You're denying yourself joy by resisting the very thing that will bring it. Living out of your own strength in order to make a difference with your life is like stopping up your cavities with sugar. So stop drawing attention to your strengths. Exult rather in your weaknesses. And then the pipes of your heart will be cleaned of all the self-sufficiency that has been clogging them up and real power will be able to flow."
What a blow to my pride, but what a comfort too! Because it means anyone qualifies. There's no minimum standard to acquire God's power. All it takes is weakness. And everyone has that. The only thing that disqualifies you is thinking you (yourself) qualify. The only thing that qualifies you is knowing you don't. Think God can't use you? There you go: he can. Think God needs you and that you have a lot to offer him, as I often do? Your pipes are clogged. Power won't flow.
I spent the last year of my life trying to figure out this passage. It's still sinking in and has a long way to go. But let's agree not to forget it. The world washes us in the message that strength is in strength. But the cross definitively tells us the folly and misery and emptiness of such a pursuit, giving us the supreme example of strength in weakness (2 Cor 8:9).
We sinned and thorns came up from the ground (Gen 3). Jesus wore a crown of thorns so that Paul's thorn, and ours, could strangely yet wonderfully result in good and fruitfulness. Praise his name.