John Chrysostom

"Faithful is He that calleth you, who will also do it." Observe his humility. For, because he had prayed, Think not, he says, that this happens from my prayers, but from the purpose, with which He called you. For if He called you to salvation, and He is true, He will certainly save you, in that He wills it.

Homily on 1 Thessalonians 11 ↗

Theophylact of Ohrid

Look, what humility! After he offered a prayer for them, he says: do not think that you will be saved by my prayers, but by the goodness of the God who called you. For if He called you to salvation, and He is faithful, that is, true, then He will certainly do according to His will and save you.

Commentary on 1 Thessalonians ↗

Thomas Aquinas

Then when Paul says: He who calls you is faithful, he expresses the hope that his prayer will be heard, as if saying: it will come about as I hope, for He who calls you will do it, that is, He will accomplish it. "The Lord is faithful in all his words" (Ps. 145:13). "And those whom he called he also justified" (Rom. 8:30).

Commentary on 1 Thessalonians ↗