The letter closing was probably penned by Paul himself.

It was no uncommon thing in antiquity for the sender of a letter, after dictating most of it, to write the last sentence or two in his own hand. It was not usual to append one’s personal signature, although Paul does so—probably for special reasons—in 1 Corinthians 16:21; Colossians 4:18; 2 Thessalonians 3:17 (see also his signed I. O. U. in Philem. 19a). It was his autograph, not his signature, that was his authenticating mark in “all my letters” (2 Thess. 3:17); thus he starts the last section of his letter to the Galatians with the words: “See what large letters I use as I write to you with my own hand!” (Gal. 6:11).

4:21 Paul greets all the saints in Christ Jesus (for the designation cf. 1:1). Indeed, in Christ Jesus might well …