‘But he who glories, let him glory in the Lord.'
In context this is not just a general statement, true and right though such a general statement might be. It is applicable to what he has been saying. In his case his glorying is not in himself but in the Lord. All that he has achieved has been through Him. He is glorying in what the Lord has achieved. So let all behave in the same way. Let all who would glory have something to glory in that the Lord has done through them. Woe to those who merely glory in what they themselves have done.
The same quotation, probably based on Jeremiah 9:24, appears in 1 Corinthians 1:31. The idea there is that our glory should be only in Jesus Christ in Whom is total salvation, which is why God has chosen the weak and foolish things of the world to put to shame the mighty. There the thought is that all credit should go to the Lord and not man. Salvation is His work and not dependent on man's ability. In contrast here he thought is that those who are Christ's should only glory in what He achieves through them.