John Chrysostom

"The salutation of me Paul with mine own hand, which is the token in every Epistle: so I write. The grace of our Lord Jesus Christ be with you all." This he says that he writes in every Epistle, that no one may be able to counterfeit them, his subscription being subjoined as a great token. And he calls the prayer a salutation, showing that everything they then did was spiritual; even when it was proper to offer salutation, the thing was attended with advantage; and it was prayer, not merely a symbol of friendship. …

Homily on 2 Thessalonians 5 ↗

Theophylact of Ohrid

He calls the greeting a prayer, showing by this that even when it was necessary to greet, this was accomplished with spiritual benefit and prayer, and was not a simple sign of friendship. So that letters would not be composed in my name by any deceivers (and this, as we said at the beginning, many were doing), I, he says, write the greeting with my own hand. "In every epistle," which will perhaps be sent to you, or in every epistle, to whomever it may be sent.

Commentary on 2 Thessalonians ↗

Thomas Aquinas

"The salutation of Paul, with my own hand." He says this because of unbelievers who were twisting his epistles. "See with what kind of letters I write to you with my own hand" (Gal 6:11). "Which is the sign in every epistle. So I write."

Commentary on 2 Thessalonians ↗