To most people, Paul would have had the right to boast about his own achievements, but God had already seen the possibility and dealt with it:
2 Corinthians 12:7-9 So to keep me from becoming proud, I was given a thorn in my flesh, a messenger from Satan to torment me and keep me from becoming proud. Three different times I begged the Lord to take it away. Each time he said, ‘My grace is all you need. My power works best in weakness.’
What was this thorn in the flesh? Some people think it was sickness because Paul was sick when he first arrived in Galatia (but he also got well again).
Paul quite possibly was referring to a lifestyle that few would choose, full of the trials he had listed just a few paragraphs earlier. Paul was alive only by the grace of God:
2 Corinthians 11:24-27 Five different times the Jewish leaders gave me thirty-nine lashes. Three times I was beaten with rods. Once I was stoned. Three times I was shipwrecked. Once I spent a whole night and a day adrift at sea. I have traveled on many long journeys. I have faced danger from rivers and from robbers. I have faced danger from my own people, the Jews, as well as from the Gentiles. I have faced danger in the cities, in the deserts, and on the seas. And I have faced danger from men who claim to be believers but are not. I have worked hard and long, enduring many sleepless nights. I have been hungry and thirsty and have often gone without food. I have shivered in the cold, without enough clothing to keep me warm.
And then there was the weight of responsibility for the new believers:
2 Corinthians 11:28-29 Then, besides all this, I have the daily burden of my concern for all the churches. Who is weak without my feeling that weakness? Who is led astray, and I do not burn with anger?
And finally, there’s Paul’s inability to even escape danger by himself, ironically, in the very city in which he had been planning to persecute believers:
2 Corinthians 11:30-33 If I must boast, I would rather boast about the things that show how weak I am. God, the Father of our Lord Jesus, who is worthy of eternal praise, knows I am not lying. When I was in Damascus, the governor under King Aretas kept guards at the city gates to catch me. I had to be lowered in a basket through a window in the city wall to escape from him.