Theophylact of Ohrid

If above he testified to their "standing," then how does he now say: "what was lacking in your faith"? They did not benefit from all the teaching, and they were lacking, perhaps, knowledge about the resurrection and other similar matters. Also, there were many false teachers, which is why he said: "what was lacking." Where he feared for the faith itself, he says: "we sent Timothy to establish you" (v. 2); but here he says: "to supply," which pertains more to teaching than to establishing. …

Commentary on 1 Thessalonians ↗

Augustine of Hippo

And let not the admonition of the most blessed apostle by any means lose its place in your hearts: "If any man preach any other gospel unto you than that ye have received, let him be accursed." He does not say, More than ye have received; but, "Other than ye have received." For had he said the former, he would be prejudging himself, inasmuch as he desired to come to the Thessalonians to supply what was lacking in their faith. …

Tractates On John 98.7 ↗

Thomas Aquinas

Then when Paul says, night and day, the third effect of their relationship is explained. First, he points out the frequency of his prayer; secondly, he shows what he desires while praying (3:11). So Paul insists: We give thanks for things past; nevertheless we do not fail to pray also for future concerns, indeed, we do so night and day, that is, in adversity and prosperity. "Evening and morning and at noon I utter my complaint and moan" (Ps. 55:17). …

Commentary on 1 Thessalonians ↗