Our weakness may seem, as if it were an obstacle in the way of God’s perfecting his strength in us. Paul does not merely deny this, but maintains, on the other hand, that it is only when our weakness becomes apparent, that God’s strength is duly perfected. To understand this more distinctly, we must distinguish between God’s strength and ours; for the word my is emphatic. “My strength,” says the Lord, (meaning that which helps man’s need — which raises them up when they have fallen down, and refreshes them when they are faint,) “is perfected in the weakness of men;” that is, it has occasion to exert itself, when the weakness of men becomes manifest…

For then do we make room for Christ’s grace, when in true humility of mind, we feel and confess our own weakness. The valleys are watered with rain to make them fruitful, while in the mean time, the high summits of the lofty mountains remain dry.  Let that man, therefore, become a valley, who is desirous to receive the heavenly rain of God’s spiritual grace.

John Calvin, commentary on 2 Corinthians 12:6-10