John Chrysostom

Now what he says is somewhat of this kind, When ye lived in wickedness, and impiety, and the worst of evils, the state of compliance ye lived in was such that ye did absolutely no good thing at all. For this is, "ye were free from righteousness." That is ye were not subject to it, but estranged from it wholly. For ye did not even so much as divide the manner of servitude between righteousness and sin, but gave yourselves wholly up to wickedness. …

Homily on Romans 12 ↗

Theophylact of Ohrid

When you lived in vices, he says, you were estranged from righteousness; at that time you did not submit to it, you did not at all wish to serve it, and you freed yourselves from it. Therefore now too, serving righteousness, do not submit to sin.

Commentary on Romans ↗

Thomas Aquinas

Then he assigns the reason for this teaching, saying, for when you were the servants. In regard to this he does two things: first, he presents a reason for the teaching; second, he proves something he had presupposed, at for the wages of sin is death. The reason behind the above teaching is that the state of grace is preferable to the state of sin. For if more benefits accrue to us from the state of justice than from sin, we should be more eager to serve justice than we were to serve sin. …

Commentary on Romans ↗