According to Plato

According to Plato, an ideal polis is a state which is based on fairness and virtue. The ideal state is a type of organization that permits individuals to make the most of their potentials, serve their fellow citizens, and live in harmony with universal laws. Augustine is of the view that men can only do good if they go after the will of God. He wants people to realize that His order is superior. In short Plato has based his theory on individual and Augustine has based his theory on God.
As I comprehend, Augustine believed that an ideal city is to be based on the laws of God. So in order to make Plato’s ideal polis to resemble the city of God, Augustine would make it necessary to abide by the rules provided by the Divine. Plato’s ideal polis is ruled by the philosophers or the kings and these people must be fully educated, be able to understand the difference between the reality and appearances. Augustine would have made compulsory for the ruling class to get complete knowledge of the religion as well so as to govern the state according to the rules set by the God.
Similarly, Augustine would have promoted feelings of love among the individuals, because according to Augustine, if we love our neighbors, we are expressing love of God. Plato thinks that in an ideal state every citizen is supposed to work honestly according to their place designated in the society and Augustine would have inculcated among the individuals that they should also follow the God’s will in order to receive His grace. For Augustine a just society is not enough but rather he would try to explain it to the people that religion is equally important.