John Chrysostom

"Salute Apelles, approved in Christ." There is no praise like this, being unblamable, and giving no handle in the things of God. For when he says, "approved in Christ," he includes the whole list of virtues. And on what ground does he nowhere say my Lord such an one, my Master this? It is because these encomiums were greater than those. For those are mere titles of rank (timhj), but these are of virtue. And this same honor he paid them not at random, or as addressing several of inferior virtue with the high and great characters. …

Homily on Romans 31 ↗

Theophylact of Ohrid

That is, blameless and irreproachable in all things, and some say that this is said in distinction from some other Apelles who was not deserving of approval. These were probably not the same as the former ones, which is why he did not mention them by name.

Commentary on Romans ↗

Thomas Aquinas

Then he says, greet Apellas, approved in Christ, perhaps through certain tribulations: when he has tried me, I shall come forth as gold (Job 23:10). Then he says, greet those who are of the house, i.e., of the family, of Aristobulus, in whose house many believers were gathered whom he does not greet, perhaps because he was absent for some reason.

Commentary on Romans ↗