2 Corinthians 12:6

Even if I should choose to boast, I would not be a fool, because I would be speaking the truth. But I refrain, so no one will think more of me than is warranted by what I do or say.

(v6, NIV)

Though if I should wish to boast, I would not be a fool, for I would be speaking the truth. But I refrain from it, so that no one may think more of me than he sees in me or hears from me.

(v6, ESV)

For if I do wish to boast I will not be foolish, for I will be speaking the truth; but I refrain from this, so that no one will credit me with more than he sees in me or hears from me.

(v6, NASB)

If Paul were to boast, he would actually look good in foolish eyes. He would be able to puff himself up with things other people view as worthy of boasting. Paul has many things he could easily boast about. Also, sometimes when you boast about something, people do not believe you at first; especially the more spectacular the thing is that you are boasting in. He would not be a fool because he would be boasting about things that are true. Unfortunately, some people who boast are boasting in untrue things. Sometimes people will recount certain stories and exaggerate certain details. However, if you're caught in your exaggeration or lying, you look foolish. Paul would not because Paul would boast in things that actually happened.

As Paul has mentioned before, he will keep from boasting. This is so that nobody will think more of him, or "credit" as the NASB version says. When we boast about ourselves, we are elevating ourselves. If there are levels to represent what you are, the level others see you at is being raised when you boast about yourself. Notice that your actual level does not raise but your perceived level raises. Paul doesn't want that. Paul wants them to see him for who he is, not for an augmented picture of himself. He doesn't want to hide behind a mask and pretend to be someone he's not. He wants his actions and words to speak for themselves.