John Chrysostom

"That ye be not quickly shaken," he says, "nor yet be troubled, either by spirit, or by word, or by epistle as from us, as that the day of the Lord is now present." Here he seems to me to intimate that certain persons went about having forged an Epistle, as if from Paul, and showing this, said that the Day of the Lord is at hand, that thence they might lead many into error. …

Homily on 2 Thessalonians 3 ↗

Theophylact of Ohrid

"And not to be troubled either by spirit," that is, through prophecy. For some, hypocritically explaining prophecy, were deceiving the people, saying that the coming of the Lord had already occurred. "Nor by word," that is, by teaching expressed in living language (orally). "Nor by letter, as if sent by us." Deceivers confirmed what they were saying with forged epistles, as if sent by Paul. "As though the day of Christ is already at hand." Do not be alarmed, he says, that the day of the Lord has come, that is, the coming of Christ.

Commentary on 2 Thessalonians ↗

Thomas Aquinas

But of what is he trying to persuade them? "That you be not easily moved from your sense." It is one thing to be moved and another to be terrified. Now the one who is moved from his sense loses what he had previously held fast. The Apostle says, in effect, "do not easily let go my teaching." "Who easily believes is light of heart" (Sir 19:4). But terror is a kind of trepidation with fear of what opposes one. And so he says "nor be terrified." "The sound of terror always in his ears" (Job 15:21). …

Commentary on 2 Thessalonians ↗