“There are two paths, one of life and one of death, and the difference between the two paths is great. This then is the path of life. First, love the God who made you, and second, your neighbor as yourself. And whatever you do not want to happen to you, do not do to another. This is the teaching relating to these matters: Bless those who curse you, pray for your enemies, and fast for those who persecute you. For why is it so great to love those who love you? Do the Gentiles not do this as well? But you should love those who hate you—then you will have no enemy. Abstain from fleshly passions. If anyone slaps your right cheek, turn the other to him as well, and you will be perfect. If anyone compels you to go one mile, go with him two. If anyone takes your cloak, give him your shirt as well. If anyone seizes what is yours, do not ask for it back, for you will not be able to get it.”
Didache (The Teaching of the Twelve Apostles) 1.1-4.