Augustine of Hippo (354 A.D. to 430 A.D.) is one of the most complex authors of his era. An African, he is part of the Roman Empire; a Christian, he was educated in pagan philosophy. His books deal with a wide variety of topics, and sometimes with more than one topic at the same time. He is defends the Christians who have been blamed for the fact that the Goths from northern Europe attacked and trashed Rome in 410 A.D.; the Roman polytheists said that the presence of Christians had angered the pagan gods and weakened the city. Augustine points out that the City of Rome would have been trashed even worse if the Christian churches hadn't protected both the pagan and Christian citizens of Rome.
Augustine uses the metaphor or image of two cities in his writings, but it is difficult to exactly define what they represent. The two cities are, of course, not actual physical cities, but symbols, for those who embrace Christianity as compared to those who cling to paganism. Alternatively, the can be interpreted, not as those two groups of people, but rather as two sets of ideas, and the interplay between them.
He is realistic enough to say that you will never have a society which is 100% pagan or a society which is 100% Christian, so the two groups have to cooperate, and they have common goals which will help them do this. Augustine pleads for tolerance: the pagans should stop killing the Christians, and stop blaming them for Rome's misfortunes, and simply allow the Christians to live peacefully within the Empire. Augustine echoes, in this way, the Emperor Constantine, who essentially founded the idea of tolerance, when he decided that Jews, Christians, and pagans would all be allowed to study their ideas in Roman society.
Augustine also says that simply being a member of the church doesn't guarantee that a person really has a Christian spiritual desire for peace. Being a Christian is studying and believing the distinctive ideas presented by Jesus (that each human life is valuable, that peace is better than war, etc.); being a Christian is different than merely being a member of a church.
As part of the Christian tradition, Augustine embraces the whole idea of God's unearned favor toward people; nobody earns God's gifts, and the thing that will really get a person into trouble is if she or he thinks that he or she is good enough to earn God's favor. Admitting that you're not perfect (says Augustine) is the basis for Christianity. These concepts apply equally to all people, making Christianity a user-friendly religion.
Augustine criticizes the pagans for thinking that earthly peace can be achieved by human intellect and abilities. Rather, he says, peace has a spiritual origin beyond human beings. Human beings, trying to use their own powers, are imperfect and insufficient to create peace. Augustine is pointing out that human reason is good and powerful, but it is not perfect, and there are somethings that human reason can't do. We need to be honest enough with ourselves to admit that we have limits. Centuries later, Augustinian philosophers, who formed one part of the Scholastic movement in the Middle Ages, will apply these ideas in a new setting.
Augustine points out the need the balance both the Christian impulse for societal involvement (helping the poor, founding schools, etc.) and the Christian impulse for meditation and contemplation: both are good, he says, but we should not have too much of either.
The future of human history is, then, societies continually trying to find peace and justice, and sometimes succeeding more, and sometimes succeeding less, depending on how the mix of people works together.
