But in the Gentile all this takes place inwardly, in himself and in his nature. There the law is heard by him, there he attends to it, there he resolves to do it and does it.
How this happens the Apostle explains in the following verse. Here he only gives an experiential indication that the Gentile does what is lawful and does it by nature.
He does it without special guidance from above, and therefore is a law to himself. It must be noted that when the Apostle says "by nature," he does not mean by natural necessity.
He means by the natural powers that exist by nature. With these powers men both recognize what is lawful, resolve upon it, and do it, while preserving full freedom either to follow it or not.
"When he says, 'by nature,' he means according to natural reason" (Chrysostom). They have "conviction in their thoughts, having imprinted in their hearts not letters, but the deed.
Instead of the law, they use conscience as a witness to the good, together with natural thoughts" (blessed Theophylact).
Likewise, when the Apostle says that they are a law to themselves, he does not assign them the right to act however anyone wishes.
He only teaches that the law did not come to them from outside. In its quality, however, it is as unalterable as the written law.
It is the same in all and equally binding upon all. For nature is unchanging and one in all, and he who acts against nature destroys nature.
But how did the Apostle earlier portray the Gentiles as so corrupt that one would not even think of saying that they do by nature the things of the law?
There he portrayed them as they became by their own will, having fallen into the lusts of the flesh. Here he points to what they ought to be according to nature.
They ought to do what is lawful, what is written in their nature. There the dominant character of paganism was presented, showing to what moral degradation it had fallen.
Here he says that, despite all this, there were also some who did what is lawful. These latter serve to expose those who knew, were able to do what is lawful, and did not do it.
If there were no doers of the law, the lawless could say: What is to be done? Evidently we are so constituted.
Here the Apostle gives it to be understood that they are not constituted to be impious and corrupt. On the contrary, they are constituted to do what is lawful.
In this place the Apostle does not need the proposition that all Gentiles do all lawful things. He needs only this: they do what is lawful.
From this he draws the conclusion that they have their own law acting within them.