13 So also came there to be courts of justice, and so were penalties defined, as Paul accordingly observes. For since many of the Gentiles were ready to controvert this, and to say, “How will God judge mankind who lived before Moses? He did not send a lawgiver; He did not introduce a law; He commissioned no prophet, nor apostle, nor evangelist; how then can He call these to account?” Since Paul therefore wished to prove that they possessed a self taught law; and that they knew clearly what they ought to do; hear how he speaks; “For when the Gentiles who have not the law, do by nature the things contained in the law, these having not the law, are a law unto themselves; which show the work of the law written in their hearts.” But how without letters?
“Their conscience also bearing witness, and their thoughts the meanwhile accusing, or else excusing one another. In the day when God shall judge the secrets of men by Jesus Christ according to my gospel.” And again; “As many as have sinned without law, shall perish without law; and as many as have sinned in the law, shall be judged by the law.” What means, “They shall perish without law?” The law not accusing them, but their thoughts, and their conscience; for if they had not a law of conscience, it were not necessary that they should perish through having done amiss.
For how should it be so if they sinned without a law? But when he says, “without a law,” he does not assert that they had no law, but that they had no written law, though they had the law of nature. And again; “But glory, honour, and peace, to every man that works good, to the Jew first, and also to the Gentile.”
Source: Homilies on the Statues (New Advent)