John Chrysostom

In this place he has also veiled over something. "But now once more in the end of the world." Why "at the end of the world"? After the many sins. If therefore, it had taken place at the beginning, then no one would have believed; and He must not die a second time, all would have been useless. But since later, there were many transgressions, with reason He then appeared: which he expresses in another place also, "Where sin abounded, grace did much more abound. …

Homily on Hebrews 17 ↗

Theophylact of Ohrid

For, he says, if it were necessary for Him to offer sacrifices many times, then it would have been necessary for Him to die many times as well, by virtue of the fact that He had to offer His own Blood. Here he also reveals a certain mystery of why He appeared "at the end of the ages," after a multitude of sins. For if His death had occurred at the beginning, when sin was not yet so widespread, then no one would have believed, and a second time He was not obliged to die; consequently, everything would have seemed useless. …

Commentary on Hebrews ↗

Thomas Aquinas

470. – Then when he says, For then he ought to have suffered repeatedly since the foundation of the world, he proves that it would have been unbecoming to do the same thing in the New Testament, because the greatest impropriety would follow, because since Christ entered with his blood, it would follow that He would have to have suffered frequently from the foundation of the world. For this is not the case in the sacrifices of the Old Testament, because they were offered for the sins of the children of Israel. …

Commentary on Hebrews ↗