Form, not strength of conviction
As I understand Sullivan to say, this added oomph derives from the non-provisional nature of the religious conviction. The flipside to this is that non-religious worldviews are provisional and subject to change; in turn, more political leeway is granted those that disagree with us. The obvious counter, of course, is that I hold many beliefs as non-provisional. I hold that murder is wrong, and this belief is absolutely non-provisional and non-negotiable. It doesn’t seem, though, that our imposition of this belief through politics is any sense theocratic, or similar to the logic of Christianism.
On many of the moral propositions on which there’s broad agreement, then, Sullivan’s distinction is simply untenable. Just because I believe something non-provisionally doesn’t mean it’s of a kind with Christianist politics.
So where do we locate the difference between capital-L Liberal politics and Christianist ones? Let’s start with some of the hallmarks of Christianist political activity: pornography, decency (cursing and the like), and homosexuality*. All of these things have in common a “thick” conception of the self toward which all people ought to tend. In short, the Christianist position on all of these is one of eudaimonia. There is an ideal, thick conception of the self, and the law ought to be used to encourage movement toward that ideal. You should be straight, virtuous, chaste, etc., regardless of your good intentions or the consequences of your actions. That’s the Christianist position, and it is distinctly eudaimonian. By contrast, Liberal conceptions of the political sphere (notably Rawls’s, to whom this post is obvious indebted) begin from the presupposition that these thick conceptions of the self (the Rawlsian “comprehensive doctrine”) should not be enforced through political coercion. The politically Liberal notion of politics is a space in which people can pursue happiness so long as others aren’t harmed; the Christianist notion tries to shrink the boundaries of that space to coincide perfectly with the Christianist notion of the virtuous self.
* Bracketing abortion for the sake of simplicity