John Calvin Commentary 1 Thessalonians 5:15

"See that none render unto any one evil for evil; but always follow after that which is good, one toward another, and toward all." — 1 Thessalonians 5:15 (ASV)

See that no one render evil for evil. Because it is difficult to observe this precept, due to the strong tendency of our nature towards revenge, he therefore urges us to be on our guard. For the word see denotes anxious care. Now, although he simply forbids us from retaliating against each other by inflicting injuries, nevertheless, there can be no doubt that he also intended to condemn every disposition to do injury.

For if it is unlawful to render evil for evil, every disposition to injure is culpable. This doctrine is distinctive of Christians—not to retaliate for injuries, but to endure them patiently. And so that the Thessalonians would not think that revenge was prohibited only towards their fellow believers, he expressly declares that they are to do evil to no one.

For particular excuses are often brought forward in some cases. “What! Why should it be unlawful for me to avenge myself on someone who is so worthless, so wicked, and so cruel?” But since vengeance is forbidden to us in every case, without exception, however wicked the person who has injured us may be, we must refrain from inflicting injury.

But always follow benignity. By this last clause he teaches that we must not merely refrain from inflicting vengeance when anyone has injured us, but must cultivate beneficence towards all. For although he means that it should first be practiced mutually among believers, he afterwards extends it to all, however undeserving they may be, so that we may make it our aim to overcome evil with good, as he himself teaches elsewhere (Romans 12:21). The first step, therefore, in the practice of patience, is not to avenge injuries; the second is to bestow favors even upon enemies.