A.T. Robertson Commentary 1 Corinthians 1:31

"that, according as it is written, He that glorieth, let him glory in the Lord." — 1 Corinthians 1:31 (ASV)

That (ινα). Probably ellipse (γενητα to be supplied) as is common in Paul's Epistles (2 Thessalonians 2:3; 2 Corinthians 8:13; Galatians 1:20; Galatians 2:9; Romans 4:16; Romans 13:1; Romans 15:3). Some explain the imperative καυχασθω as an anacoluthon. The shortened quotation is from Jer 9:24. Deissmann notes the importance of these closing verses concerning the origin of Paul's congregations from the lower classes in the large towns as "one of the most important historical witnesses to Primitive Christianity" (New Light on the N.T., p. 7; Light from the Ancient East, pp. 7, 14, 60, 142).