Paul is saying this because there is no power against the truth. They cannot reprove someone who is living a good life, but only someone who is an enemy of the law. This power will come to nothing if people have done what is good.
"For we can do nothing against the truth, but for the truth." For that he may not seem [merely] to be gratifying them, (for this is what one who was void of vain-glory might do,) but to be doing what the nature of the thing demanded, he added this, "for we can do nothing against the truth." "For if we find you," he says, "in good repute, having driven away your sins by repentance and having boldness towards God; we shall not be able thereafter, were we never so willing, to punish you, but should we attempt it even, God will not work with us. For to this end gave He us our power that the judgment we give should be true and righteous, not contrary to the truth."
Seest thou how in every way he can, he makes what he says void of offensiveness, and softens the harshness of his menace? Moreover as he has eagerly endeavored this, so is he desirous also to show that his mind was quite joined to them.
[AD 384] Ambrosiaster on 2 Corinthians 13:8