Isaiah 52:11

It says, "you who carry the vessels of the Lord be separated and depart from their midst, says the Lord." "Separate yourself" from earthly deeds; "separate yourself" from the desire of the world.… Moreover, we say to be set apart not from places but from deeds, not from regions but from ways of life.

When the Romans were on the point of making a campaign against Jerusalem, all those who had accepted the message exiled themselves in other cities, for they had learned of the misfortunes that would strike Jerusalem. Thus, it is the Lord in person who commanded them to do this: "But when you see Jerusalem surrounded by armies, then know that its desolation is near," and again: "Then, let those who are in Judea flee to the mountains, let him who is on the housetop not come down to take anything out of his house." Therefore, because they knew these events in advance, they went out and escaped the misfortunes of the siege of the city, while the Savior in person guided them, directed their steps toward the Gentiles and assembled the church derived from the Gentiles. Since we also have learned of these events, let us, therefore, flee disbelief; let us persevere in the faith; let us guard the commandments of God and let us proceed on the right path, with the Lord Jesus to guide our way.

What else does touching anything unclean mean, anyway, but consenting to sins? And what else does going out from there mean, but doing what is involved in rebuking the bad, to the extent that this can be done, taking account of each person's status and position, without damage to peace? It has displeased you that someone sinned; you have taken care not to touch an unclean thing. You have challenged, rebuked, warned, even administered, if the matter called for it, a suitable penalty that does no violence to unity; you have gone out from there.…When Scripture thunders at us that we must withdraw from the wicked, we are only being required to understand that we must withdraw in our hearts. Otherwise we may commit a greater evil by separating too simply the good from the bad. This is what the Donatists have done.… To understand what he said, I pay attention to what he did.… What he said was "Depart." … What I am asking is, did he depart physically from such people? I find that he did not. So he must have understood his own words differently. Because, of course, he himself would be the first to do what he commanded.… God, after all, could not blame him for his own sins, because he did not commit any; or for other people's, because he did not approve of them; or for indifference or neglect, because he did not keep quiet; or for pride, because he remained steadily in the unity of God's people.