John Chrysostom

Then again He fences them in with great terror. For think not at all, saith He, that the thing stops at this, that is, at the condemnation of the multitude; nay, for all that do wickedly in such things shall suffer the utmost punishment. And He said not, "ye," partly in order to instruct our whole race, partly to make His saying the less burdensome. …

Homily on the Gospel of Matthew 42 ↗

Theophylact of Ohrid

Here He strikes fear into our hearts, that we will give an account for even a careless word, that is, any lying, slanderous, indecent, or mocking word. Then He brings forward testimony from Scripture, lest He appear to be speaking His own words. "By thy words thou shalt be deemed righteous, and by thy words thou shalt be condemned" (Job 15:6; cf. Ecclesiastes 10:12).

Commentary on Matthew ↗

Jerome

But I tell you that every idle word that men speak, they shall give an account thereof in the day of judgment. For by your words you shall be justified, and by your words you shall be condemned. This also aligns with what was said before. …

Commentary on Matthew ↗

Gregory the Dialogist

I admonish you together with myself that we refrain from idle speech, that we avoid speaking uselessly. Insofar as we are able to restrain our tongue, let not words flow away into the wind, since the Judge says: "Every idle word that men have spoken, they will render an account of it on the day of judgment." An idle word is one that lacks either the usefulness of righteousness or the reason of just necessity. …

Forty Gospel Homilies, Homily 6 ↗

Thomas Aquinas

I tell you... The Lord rebuked them for the gravity of their sin and their malice; here he warns them about the future judgment, which we hold by faith: "Be afraid of the sword, for anger brings the punishment of the sword; for know that there will be a judgment" (Jb 19:29); "He will make room for every act of mercy; everyone will receive in accordance with his deeds" (Sir 16:14). …

Commentary on Matthew ↗