Theo Hobson argues that John Milton was a great (and underrated) theorist of a distinctly Christian liberalism. He was a strong proponent of a secular state, but also of a Protestant ethos throughout society:

So was he an early “secular liberal”? Not in the dominant contemporary sense, which assumes that politics should be post-religious. He thought it should be post-ecclesial, but that liberal Protestant Christianity was the necessary foundation of a free society. This must be the national ideology, but it must not be identified with any religious institution. In effect, he was inventing the American approach to church-state relations.

Hobson apparently has a book just out on the subject that looks like it would be worth checking out.