This dissertation is an interpretative study of Saint Augustine's City of God. I argue that we need to examine three communities in City of God in order to better appreciate Augustine's insight on politics, and in particular his assessment of the relationship between the city and the philosophers, and between politics and reason. Specifically, this dissertation is based on the observation that three communities stand out in Augustine's discussion of civitas terrena: Rome, the first human communion of Adam and Eve, and Babylon; each of them points to one unique feature of earthly politics, and all three together form a whole picture of Augustine's understanding of politics: Augustine's critique of Rome comprises a critique of the triple lust (libido); while Babylon for him is a city of confusion (civitas confusionis); both lust and confusion find their origin in the corruption of the natural communion of Adam and Eve. Moreover, in discussing each of them, Augustine has a particular interest in engaging with philosophers' opinions and the role of reason in each of the communities. The City of God as a whole therefore could be read not only as Augustine's response to those who accused Christianity for the sack of Rome, but also as Augustine's evaluation of politics in general, and in particular of the question regarding the potentiality and limits of politics in terms of bringing the soul into the right order.