John Chrysostom

When, pray, was that? Before Moses. See how he sets himself to show that it, both by the things it did, and the things it did not do, weighed down human nature. For when "I was alive without the Law," he means, I was not so much condemned. "But when the commandment came, sin revived, and I died." This seems indeed to be an accusing of the Law. But if any one will look closely at it, it will be seen to be even an encomium of it. For it did not give existence to sin that before was not, but only pointed out what had escaped notice. …

Homily on Romans 12 ↗

Theophylact of Ohrid

Before Moses, he says, I lived without the law, which is why I was not subject to strict condemnation (here in his own person he means human nature); but when the commandment came, it was revealed that sin is sin: for although people sinned even before, they were not aware of it. And this is precisely the benefit of the law, that it made people conscious that they are sinning.

Commentary on Romans ↗

Methodius of Olympus

Let us see, then, what it is that we have endeavoured to say respecting the apostle. For this saying of his, "I was alive without the law once," refers to the life which was lived in paradise before the law, not without a body, but with a body, by our first parents, as we have shown above; for we lived without concupiscence, being altogether ignorant of its assaults. …

Methodius From the Discourse on the Resurrection ↗

Thomas Aquinas

But the condition so far as man was concerned is indicated when he says, and I lived some time without the law. This can also be understood in two ways; in one way with respect to the fact that it seemed to man that he was alive, so long as he did not know that sin was that by reason of which he was dead: you have the name of being alive, but you are dead (Rev 3:1). Or in another way, this is said in comparison to the death which followed by occasion of the law. …

Commentary on Romans ↗