104 And for the rest, most dear brethren, consider that Jesus suffered without the gate, and do you go forth out of this earthly city, for your city is Jerusalem which is above. Let your conversation be there, that you may say: “But our conversation is in heaven.” Therefore did Jesus go forth out of the city, that you going out of this world may be above the world. Moses alone, who saw God, had his tabernacle without the camp when he talked with God; and the blood indeed of the victims which were offered for sin, was brought to the altar, but the bodies were burnt without the camp; for no one placed amidst the evil of this world can lay aside sin, nor is his blood accepted of God, except he go forth from the defilement of this body.
Source: Letters (New Advent)