John Calvin Commentary 2 Thessalonians 3:15

"And [yet] count him not as an enemy, but admonish him as a brother." — 2 Thessalonians 3:15 (ASV)

Regard him not as an enemy. He immediately adds a softening of this rigor, for, as he elsewhere commands, we must take care that the offender not be swallowed up with sorrow (2 Corinthians 2:7), which would happen if severity were excessive. Therefore, we see that discipline ought to be used in such a way as to promote the welfare of those on whom the Church inflicts punishment.

Now, it is inevitable that severity will cause distress when it goes beyond proper limits. Therefore, if we wish to do good, gentleness and mildness are necessary, so that those who are reproved may know that they are nevertheless loved. In short, excommunication is not intended to drive people from the Lord’s flock, but rather to bring them back when wandering and going astray.

We must observe, however, by what sign he wishes brotherly love to be shown — not by allurements or flattery, but by admonitions; for in this way, all who are not incurable will feel that concern is shown for their welfare. Meanwhile, excommunication is distinguished from anathema: for concerning those whom the Church marks out by the severity of its censure, Paul admonishes that they should not be utterly cast away, as if they were cut off from all hope of salvation; but efforts must be made so that they may be brought back to a sound mind.