Jerome

So it is not the will of your Father who is in heaven that one of these little ones should perish. He refers to the previously stated proposition, of which he said: See that you do not despise one of these little ones; and he teaches this by means of a placed parable, so that the little ones may not be despised. And in what he says: It is not the will of your Father that one of these little ones should perish: whenever one of these little ones perishes, it is shown that it is not by the will of the Father.

Commentary on Matthew ↗

Gregory the Dialogist

We must consider, my brothers, why the Lord declares there is more joy in heaven over converted sinners than over the just who stand firm, unless it is what we ourselves know through daily experience of observation: that very often those who know themselves weighed down by no burden of sins do indeed stand in the way of justice, commit no unlawful acts, yet do not anxiously long for the heavenly homeland, and grant themselves as much use of lawful things as they remember having committed no unlawful ones. …

Forty Gospel Homilies, Homily 34 ↗

Thomas Aquinas

He concludes therefore: so it is not the will of your Father, who is in heaven, that one of these little ones should perish. He says less and signifies more, because his will is that they be saved; 1 Tim 2:4: who will have all men to be saved. For if he did not will it, he would not send angels. Ezek 18:23: is it my will that a sinner should die? says the Lord.

Commentary on Matthew ↗