25 It is now surely plain to your discernment, in what manner all such testimonies of the Scriptures are to be received; so that when Scripture says, that we must depart from the wicked, we are bid to understand this in no other sense, but that we depart in heart; lest by the separation from the good, we commit a greater evil than we shrink from in the union of the wicked, as these Donatists have done. But if they were truly good, and so had reproved the wicked, and not rather being themselves wicked, had defamed the good, they would for peace sake bear with any, be they who they might, seeing they have received the Maximianists as sound, whom they condemned before as lost.
Undoubtedly the Prophet has said plainly, “Depart ye, go out from thence, and touch not the unclean thing.” But that I may understand what he said, I pay attention to what he did. By his own deeds he explains his words. He said, “Depart ye.” To whom did he say so? To the righteous of course. From whom did he bid them depart? From sinners and wicked men of course. I ask then, did he depart from such himself? I find that he did not. So then he understood it in another sense. For surely he would be the first to do what he enjoined.
He departed from them in heart, he rebuked and reproved them. By keeping himself from consenting to them, he “did not touch the unclean thing;” but by rebuking them he “went out” free in the sight of God; and to him God neither imputes his own sins, because he sinned not; nor the sins of others, because he approved them not; nor negligence, because he kept not silence; nor pride, because he continued in unity. So then, my Brethren, how many soever you have among you, who are still weighed down by the love of the world, covetous, or perjured persons, adulterers, spectacle hunters, consulters of astrologers, of fanatics, of soothsayers, of augurs and diviners, drunkards, sensualists, whatever there is of bad that you know you have among you; show your disapprobation of it all as far as you are able, that you may in heart “depart;” and reprove them, that you may “go out from them;” and consent not to them, that “ye touch not the unclean thing.”
Source: Sermons on Selected Lessons of the New Testament (New Advent)