Paul reluctantly enters the game of boasting his opponents were playing. Like a father who has promised his daughter in marriage, he wants to present the church as a pure bride to Christ. His fear is that, as the serpent deceived Eve, their minds will be corrupted from simple, pure devotion to Christ. These intruders preach another Jesus, a different spirit, and a different gospel, and the Corinthians tolerate it.
Paul had preached to them free of charge, and he insists he will keep doing so (11:7-12). He names them false apostles, deceitful workers, masquerading as apostles of Christ. Satan himself masquerades as an angel of light, so it is no surprise that his servants appear as servants of righteousness.
Speaking as a fool, Paul matches their credentials: Hebrew, Israelite, descendant of Abraham. Then he turns boasting on its head. As a servant of Christ, he lists far greater labors, prisons, and harsh treatment. He faced shipwrecks and dangers from rivers, robbers, and false brothers. He knew toil, lack of sleep, hunger, thirst, and cold. Each day he felt concern for all the churches. If he must boast, he will boast of his weakness. His final example is humbling: he was let down in a basket through a window by the wall of Damascus (11:33).