John Chrysostom

"In flaming fire rendering vengeance to them that know not God, and to them that obey not the Gospel of our Lord Jesus." If they that have not obeyed the Gospel suffer vengeance, what will not they suffer who besides their disobedience also afflict you? And see his intelligence; he says not here those who afflict you, but those "who obey not." So that although not on your account, yet on His own it is necessary to punish them. …

Homily on 2 Thessalonians 2 ↗

Theophylact of Ohrid

Those whom he previously called those who "trouble" them, he now calls those who "know not God." By this he very wisely teaches that vengeance will come in any case. For if not for your sake, then for His own sake He will certainly take vengeance on His enemies. …

Commentary on 2 Thessalonians ↗

Thomas Aquinas

Then when he says "in a flame of fire," he treats of both, namely, the punishment of the wicked and the reward of the good; but he shows the harshness, justice, and eternity of the punishment of the wicked. He says, therefore, "giving vengeance," i.e., judging those who are to be punished "in a flame of fire," consuming the face of the earth, enveloping the condemned, and driving them away forever. "Fire goes before him" (Ps 96:3). …

Commentary on 2 Thessalonians ↗