19 Yet in the mysterious sense of this passage, Brethren, we recognise what is expressed most plainly in other places of the sacred books, that there are within the Church both good and bad, as I often express it, wheat and chaff. Let no one leave the floor before the time, let him bear with the chaff in the time of threshing, let him bear with it in the floor. For in the barn he will have none of it to bear with. The Winnower will come, who shall divide the bad from the good.
There will then be a bodily separation too, which a spiritual separation now precedes. In heart be always separated from the bad, in body be united with them for a time, only with caution. Yet be not negligent in correcting those who belong to you, who in any way appertain to your charge, by admonition, or instruction, by exhortation, or by threats. Do it, in whatsoever way ye can. And because ye find in Scripture and in the examples of Saints, whether of those who lived before or after the coming of the Lord in this life, that the bad do not defile the good in unity with them, do not on this account become slow in the correction of the bad.
In two ways the bad will not defile you; if you consent not to him, and if you reprove him; this is, not to communicate with him, not to consent to him. For there is a communication, when an agreement either of the will or of the approbation is joined to his deed. This the Apostle teaches us, when he says, “Have no communication with the unfruitful works of darkness.” And because it was a small matter not to consent, if negligence in correction accompanied it, he says, “But rather reprove them.” See how he comprehended both at once, “Have no communication, but rather reprove them.” What is, “Have no communication”? Do not consent to them, do not praise them, do not approve them. What is, “But rather reprove them”? Find fault with, rebuke, repress them.
Source: Sermons on Selected Lessons of the New Testament (New Advent)