David says in Psalms 21:5, speaking of “the king,” “His glory is great in thy salvation: honour and majesty hast thou laid upon him.” This is quite remarkable. David had many earthly and fleshly things to glory of. If he wanted to boast about something of worldly value he had no lack of material. He was “of a beautiful countenance,” he was “cunning in playing, and a mighty valiant man, and a man of war, and prudent in matters, and a comely person, and the LORD is with him.” [1 Sam 16:12; 18] The Bible even says that David had a perfect heart with The LORD. [1 Sam 13:14, Acts 13:22, 1 Ki 11:4; 15:3]
Our glory should be in nothing except Jesus Christ’s cross. It is through Jesus’ death and resurrection that we find life, eternal life. As Paul said in Galatians 6:14, “But God forbid that I should glory, save in the cross of our Lord Jesus Christ, by whom the world is crucified unto me, and I unto the world.”
If we boast/glory, let it be in Jesus’ cross. Or let it be in His character; as Jeremiah says,
"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." [Jer 9:24]
Or another reason to boast is because of our weaknesses. As Paul states,
"If I must needs glory, I will glory of the things which concern mine infirmities." [2 Cor 11:30]
Our glory should be great in one thing: Jesus Christ and the knowledge of Him. And through our weaknesses Jesus’ “strength is made perfect” so to boast/glory in our weaknesses is in effect to boast/glory of Jesus’ strength: that though we are weak and beggarly, poor and needy, yet Jesus Christ uses us and strengthens us, the most glorious example of all in this being salvation, because we are utterly incapable of saving ourselves.
We ought to spend our energy is boasting of Christ: in His strength, in His character, in His victory that He obtained through the cross. It is foolish to boast of anything else.