There is some discussion of the relationship between Church and State in the combox for Thomas Storck's Liberalism and the Absence of Purpose. My short contribution: "...while it may be somewhat useful to think of the division between the sacred and the secular in terms of supernatural and natural (especially with respect to the limits of secular authority), this can lead to be problems if we do not consider the underlying reason why this division is apt, namely the goods themselves. The political common good is communal well-being, or the virtuous life of the community – civic friendship. All friendships are open to something higher serving as a source of unity joining friends together – in this case, the higher good is God Himself. While secular authority may not be able to directly bring about man’s salvation through the imparting of grace, a Christian polity can nonetheless promote conditions which facilitate the observance of the Christian life (for example, removing obstacles)."