John Chrysostom

After shaming them by the slavery, after alarming them by the rewards, and so exhorting them, he again rights them by calling the benefits to mind. For by these he shows that they were great evils from which they were freed, and that not by any labors of their own, and that things henceforth would be more manageable. …

Homily on Romans 11 ↗

Theophylact of Ohrid

"Thanks be to God." Our deliverance from those evils is not a matter of human strength, but of God's; which is why one must give thanks. Then you became obedient not by compulsion, but by your own heartfelt disposition; therefore do not return to the worse things, from which you withdrew voluntarily. As proof that although they themselves came, yet all this depended also on the grace of God, he added: "you delivered yourselves," that is, you were guided by God to the form of teaching. And what is the form of teaching? …

Commentary on Romans ↗

Thomas Aquinas

Then when he says, but thanks be to God, he shows the folly of obeying sin and thereby returning to the slavery of sin: first, by considering the benefit we have received. For if someone, by another's graciousness is freed from slavery, it is unfitting to subject oneself freely to slavery. Hence, since we have been freed from sin by the grace of God, it is unfitting willfully to return again to the slavery of sin. Second, by considering the state into which we were led after freedom from sin, namely, that we are slaves to justice. …

Commentary on Romans ↗