Christianity and the Public Square:

The Center for Public Justice: The Center for Public Justice is an independent, non-partisan organization dedicated to public policy research, leadership development, and civic education. With a distinctive Christian-democratic perspective, we help citizens, policymakers, and government respond to the call to pursue justice for all.
http://www.cpjustice.org

The Claremont Institute: This website has many resources in guiding church-state relationships and the articulation of a religious worldview in the public square.
http://www.claremont.org

The Williamsburg Charter: An excellent document on church-state questions hammered out by theists (including Os Guinness) and non-theists.
http://religiousfreedom.lib.virginia.edu/const/Willburg.html

For an elaboration on this document, see the book Articles of Faith, Articles of Peace, eds. Os Guinness and James Davison Hunter (Brookings Inst.).