James writes while talking about the shortness of earthly life, As it is, you boast in your arrogance. All such boasting is evil.James 4:16
Paul admits to boasting quite a few times in his second letter to Corinth, For whatever boasts I made to him about you, I was not put to shame. But just as everything we said to you was true, so also our boasting before Titus has proved true.2nd Corinthians 7:14 For even if I boast a little too much of our authority, which the Lord gave for building you up and not for destroying you, I will not be ashamed.2nd Corinthians 10:8 But we will not boast beyond limits, but will boast only with regard to the area of influence God assigned to us, to reach even to you.2nd Corinthians 10:13 I repeat, let no one think me foolish. But even if you do, accept me as a fool, so that I too may boast a little. What I am saying with this boastful confidence, I say not as the Lord would but as a fool. Since many boast according to the flesh, I too will boast.2nd Corinthians 11:16-18 But he said to me, My grace is sufficient for you, for my power is made perfect in weakness. Therefore I will boast all the more gladly of my weaknesses, so that the power of Christ may rest upon me.2nd Corinthians 12:9
Paul claims he is allowed to boast because he has endured so much. Are they Hebrews? So am I. Are they Israelites? So am I. Are they offspring of Abraham? So am I. Are they servants of Christ? I am a better one—I am talking like a madman—with far greater labors, far more imprisonments, with countless beatings, and often near death. Five times I received at the hands of the Jews the forty lashes less one. Three times I was beaten with rods. Once I was stoned. Three times I was shipwrecked; a night and a day I was adrift at sea; on frequent journeys, in danger from rivers, danger from robbers, danger from my own people, danger from Gentiles, danger in the city, danger in the wilderness, danger at sea, danger from false brothers; in toil and hardship, through many a sleepless night, in hunger and thirst, often without food, in cold and exposure.2nd Corinthians 11:22-27
How arrogant! Every parent has to teach their children that just because another child did some bad action does not mean that their child can do that bad action too. Surely, Paul should know this. Yet, Paul says he has every right to do bad things just like other people because he is better than them! That is boasting in arrogance.
This is what James condemns. God says, Thus saith the LORD, Let not the wise man glory in his wisdom, neither let the mighty man glory in his might, let not the rich man glory in his riches: But let him that glorieth glory in this, that he understandeth and knoweth me, that I am the LORD which exercise lovingkindness, judgment, and righteousness, in the earth: for in these things I delight, saith the LORD.Jeremiah 9:23, 24
Value of this Evidence
This is substantial evidence for Paul's case of being a false apostle, because it shows that Paul does not act like an apostle. Paul cannot be humble, which is what we would expect from an apostle of Jesus.