The main purpose of this paper is to analyze the relationship of the doctrine of original sin and natural law in the political teaching of Christian churches and its impact on modern liberal democratic theory. The church fathers borrowed the theory of natural law from the Stoics as the basis of their social and political teaching. One of the essential elements in the theory of Christian natural law is the doctrine of sin. It is Augustine who finally established both the doctrine of original sin and a complete Christian political theory that emphasizes on the great difference between City of God and City of Man. Because of the original sin and the fallen reality of human beings, the City of Man, along with all the secular power as such has no ultimate meaning. One of the political results of this theory is to limit the power of the secular government, which becomes the core of liberal democratic theory in the modern time.