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 …

Who is there among those destined to be born and die who will not at some point have to leave his country and suffer the loss of his estate? But by no means let Christ be forsaken. The loss of salvation and an eternal home is a much greater threat. See how the Holy Spirit cries to the prophet, "Leave. Depart. Get out of here. Do not touch any unclean thing. Get out of the middle of such things and be separate, you who bear the vessels of the Lord." And yet, those who are the vessels of the Lord and the temple of God do not go out from the middle of all this and leave so that they wouldn't have to be compelled to touch the unclean things or be polluted and corrupted with deadly food. In another place, a voice is heard from heaven, warning them ahead of time what servants of God should be doing. It says, "Come out of her, my people, so that you do not become partakers of her sins and so that you do not receive her plagues." The one who goes out and leaves does not become a partaker of the guilt. But whoever is found as a companion in her crimes will be wounded with the plagues. Therefore, the Lord commanded us in persecution to leave and to flee. He not only taught that this should be done; he did it himself. For just as the crown is given by the condescension of God and cannot be received unless the hour comes for accepting it, so those who remain in Christ but leave for a while do not deny their faith but instead bide their time. One who has fallen, however, after refusing to leave, remains only to deny his crown.

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.

Second, he enjoins the captives to flee: depart, namely, from Babylonia; touch no unclean thing, do not live among these unclean people: go out from her, my people (Rev 18:4).