John Chrysostom

"Now the just" (he says) "shall live by faith, but if any man draw back, My soul shall have no pleasure in him." This is a great encouragement when one shows that they have succeeded in the whole matter and are losing it through a little indolence. "But we are not of them that draw back unto perdition, but of them that believe to the saving of the soul."

Homily on Hebrews 21 ↗

Theophylact of Ohrid

Since we have the faith of those who are saved, not of those who draw back to perdition, he says that we are not of those who perish through "wavering" and falling into faintheartedness or doubt, but of those who are steadfast in faith, so as to preserve our own souls, that is, to acquire, safeguard, and save them. For possession, acquisition is itself "salvation."

Commentary on Hebrews ↗

Oecumenius

"But we are not of those who withdraw to destruction." Since he astonished them by saying, "and if he becomes sluggish," he says, "my soul shall have no pleasure in him." He corrects it and says: We are not those who withdraw ourselves or are sluggish, which leads to destruction: but we pertain to faith, to possess and save our souls.

The Pseudo-Oecumenian Catena on Hebrews ↗

Thomas Aquinas

550. – Then when he says, but we are not of those that shrink back and are destroyed, he applies this to his thesis. As if to say: This is the way it will be in the case of those who withdraw from the faith; but we are not the children of withdrawing unto perdition. But a person is said to be a son of anything which rules him. Thus, a person is called the son of death, when that by which he is rejected by God, rules him: 'These are they who separate themselves, sensual men, having not the Spirit' (Jude 1:19). …

Commentary on Hebrews ↗