"Two loves gave birth to two cities." (City of God, 14.28).
This simple statement is the thesis of what Augustine deemed to be one of his most important books, City of God. This book proclaimed the world view of Augustine. It showed his vision of the mission of Christians while they were dwelling on this earth and awaiting the eternal life to come after they died.
The City of God contrasts two cities: the City of God and the City of Man. The City of God is the "city of heaven", the eternal Jerusalem on the next life. The City of God is invisible to us here; it is not of this earth. It is of the other world. With distinct overtones of Neo-Platonism he taught that life on earth was little more than an illusion, and only in the City of God would people who were just come to their final resting place. In contrast, the City of Man was evil and destined to decline and fall. It was a "city of earth" that by way of example had been seen in Rome and its Empire. Although seeing the City of Man as evil, Augustine nevertheless realised that for as long as he was on this earth he was a citizen of that world. He stipulated that, although the "city of earth" was the very opposite of the "city of heaven," it was a reality that people must face. He wished that people engage the City of Man not just as an experience of pain, but as an opportunity for Christian activity.
Christians were not to reject the world entirely, but instead must try to reform it to fit more of a Christian pattern. He said that activities usually part of the City of Man - such as warfare, economic activity, education, and the rearing of children - should all be conducted in a Christian spirit. He declared that the "city" - the next empire - that arose from the ruins and emulate Rome should be based upon Christian principles. (And the Christendom that came as a Middle Ages response to his call disappointed in this regard.) In accordance with his world view that is centred on God, Augustine regarded the City of Man as evil, imperfect, and of no consequence in comparison to the City of God. Even so, he accepted that it was not about to disappear and be replaced by a perfect Kingdom of God on earth.
Augustine also taught that life every day had to be faced using the grace of Christ. Although according to Augustine most people had a destiny of never reaching the City of God after death (and on this the Church of Rome has never agreed with him), Christians were to have a life centred on God as much as possible. The fundamental pastoral point made by Augustine in writing about these two cities is that Christians live in this world but they are not of this world. They face the discomfort of being always "in transit" while living here on this earth. They are here as strangers moving through a foreign country, enjoying what the world has to offer, but always move onwards. Heaven is the true home of the Christian. It is to heaven - the City of God - that his or her affections and loyalties are primarily directed.
In his City of God and elsewhere, Augustine gave new force to the position that a Christian can never be fully at home in this world. The Christian is here a stranger. This is not in the sense that he belongs to a separate sociological group "set apart" from the world as members of an elite: the Church is not set "over against" the world in this sense. Rather, to be a Christian involves a special way of being in the world. From both his observations and his personal experience, Augustine knew and accepted that not all Christians in his church would succeed in progressing far in their Christian understanding and everyday living of the message of the Bible. Such could possibly be the observation of any bishop in any continent in any age, but the situation for Augustine may have been exacerbated by prevailing conditions. His church was smaller in number that that of the local schismatic Donatist community, and there were still pagans and pagan celebrations very much present in the area.
In Letter 140, Augustine described as "the rich of the earth" those Christians who, although they received the Eucharist, did not practice humility (humilitas), which he proposed as the most fundamental of the Christian virtues. They were still much drawn by the riches of this earth. They also lacked charity, and had little thirst for justice. Their conversion had led them to the catechumenate and to baptism, but their conversion had failed to be ongoing. In his letters Augustine regularly distinguished between the Church on earth and the Church in heaven, the latter being the "the Jerusalem in heaven" mentioned in his major work, the City of God. He stated definitively that this "Jerusalem in heaven" could not exist on earth. The Church on earth, dwelling amidst the City of Man, was a recruiting point for the City of God that a person entered after death and judgement, and was not an extension of it on the earth. On this earth, the humble Christian must live "for Christ" by being in good relationship with Jesus and also with every human being that he or she meets.
This is not a Christian faith that is solely "of the next world" but one that also is a conscious "living under grace" on this earth for the glory of God and for the mission of Christ. Even with these high ideals, Augustine was a realist who did not expect perfection of the church and of all of its individual members. He saw the church on earth as a fragile and imperfect reflection of the church in heaven, just as Augustine the preacher - he said - was a poor and unworthy reflection of Christ the Teacher.
Augustine told his congregation that he knew that there were listening to him in church persons who stole, or drank too much alcohol, or had improper sexual relationships. He also told them that he knew some of them were keen to go the pagan celebrations later that day. Even so, he accepted that conversion was a process. Especially from his own personal experience, he well knew that the forces opposing the human will could be very powerful. The baptism of a person was only the beginning to a Christian life that would improve in the eyes of God if effort were applied. The pure church, the church of saints, existed in the next life. The ministry of the church was to assist the Christian to make his or her everyday living as attuned as possible to an ongoing conversion in Christ.
Augustine and the Christian life. A page of information. (AM 721) http://www.theologian-theology.com/theologians/augustine AN2304