Plato's philosophy contains a number of features which made it attractive to early Christians; many, but not all, of them were neo-Platonists to various degrees. Certainly, Plato's concept of an immortal soul played well to them.
Dualism also appealed to the Christians. There are several different dualisms, or different axes of dualism, in Plato's thought: mind/body, material/idea, physical/metaphysical, and in the neo-Platonic schools, good/evil, man/god. Augustine played off of these concepts through the semi-metaphorical talk of two cities.
Plato also had another theory that Augustine argued in his famous work, The City of God. Here he argued that the two worlds exist concurrently. The city of earth is corrupt, full of self-love and sinners. But the city of God, which exists on earth, is a city of good, god-fearing people who love humanity. The link between Plato and Augustine is unmistakable. “Yet there are no more than two kinds of human society, which we may justly call two cities, according to the language of our Scriptures. The one consists of those who wish to live after the flesh, the other of those who wish to live after the spirit; and when they several achieve what they wish they live in peace, each after their kind,” writes Augustine, espousing a Christian dualistic theory of humanity, which was very similar to Plato’s view. Augustine was the one who made the connection clear to the Platonists. “As a Christian theologian, he puts to grateful use the Platonic concepts of ‘spiritual substance,” of evil as the privation of the good, of intuition as the basic mode of knowledge and the duality of body-soul,” writes Albert Outler. Because Augustine is able to make such a close tie between Platonic philosophy and Christianity, he made Christianity more appealing, especially to the Platonist.
