John Chrysostom

I do not, he would say, mention hell as yet, nor that great punishment, but the shame it is in this world, when ye become slaves, and slaves of your own accord too, and sin's slaves, and when the wages are such as a second death. For if before baptism, it wrought death of the body, and the wound required so great attendance, that the Lord of all came down to die, and so put a stop to the evil; if after so great a gift, and so great liberty, it seize thee again, while thou bendest down under it willingly, what is there that it may not …

Homily on Romans 11 ↗

Theophylact of Ohrid

Then he proves that it is not difficult for us not to sin. Consider, he urges, in your mind which is better: to be slaves of sin, having surrendered to it of your own will (this is what "you yield" means), and to receive as a reward death, that is, eternal punishment (for the sin of Adam produced bodily and temporal death, but sin committed now subjects a person to eternal death, that is, eternal punishment), or to obey God and receive as a reward righteousness and the blessings that flow from it?

Commentary on Romans ↗

Thomas Aquinas

In the same fashion he answers here, God forbid that we should sin, because we have been set free from the law; because if we were to sin, we would once more be reduced to the slavery of sin. Hence, he continues, know you not that to whom you yield yourselves of your own will as servants to obey voluntarily, his servants you are whom you obey. For obedience is a debt which slaves owe to their masters: slaves, be obedient to your earthly masters (Eph 6:5). Hence, when someone obeys another, he admits that he is his slave by obeying. …

Commentary on Romans ↗