Let me direct your attention to an outstanding discussion taking place at The New Republic Online between Damon Linker, author of The Theocons: Secular American Under Siege and former editor of First Things, and Ross Douthat, an associate editor of the Atlantic Monthly and author of Privilege: Harvard and the Education of the Ruling Class.

In this discussion, Douthat and Linker pretty quickly get down to a large root of their disagreement, the "liberal bargain". Linker describes the "liberal bargain" thusly:

Like every other citizen, you must be willing to accept what I call "the liberal bargain." In my book, I describe this bargain as the act of believers giving up their "ambition to political rule in the name of their faith" in exchange for the freedom to worship God however they wish, without state interference. What does this mean, in practical terms? It means that your belief in what the Roman Catholic Church believes and teaches is irrelevant, politically speaking. It simply shouldn't matter whether or not you think that justice has a divine underpinning, anymore than it should matter whether you prefer Jane Austen to Dostoevsky. In a word, liberal politics presumes that it's possible and desirable for political life to be decoupled from theological questions and disputes.

In his response, Douthat says that this "liberal bargain" is the root of the disagreement.