Miroslav Volf argues that, rightly understood, it is fundamental to Christian faith.
Separation of church and state is, I think, fundamental to the character of the Christian faith. And separation of church and state might have historically arisen out of the mistakes of Christianity, or our perception of the way in which Christianity has actually been lived out in relation to political powers. But separation of church and state – or, put differently, consideration or thinking of church or religion and the state as distinct cultural systems – I think it is fundamental to the Christian faith. It’s rooted in the very character of its having a universal kind of an appeal, of its calling individuals from the situatedness in which they find themselves to allegiance to a new community of faith, so that it introduces a fissure within the existing society by the very nature of its call. And I think in that sense, also, it advocates already a certain form separation of churches.
Now we have to understand exactly what separation of church and state means. I think that if one did understand separation of church and state simply as kind of privatisation of religion, or of the Christian faith, I think that’s a mistaken understanding of the separation. I think the better term is neutrality of the state toward all over-arching interpretations of life, and then understanding of neutrality as impartiality of the state toward all over-arching interpretations of life so that state becomes a broker, a fair broker, in the discussion and the debate that is going always on about the ways of living among over-arching interpretations of life. Once you understand it in those terms, then you can see Christian faith, religion, as a separate cultural system, but not unrelated to the political concerns that we have. To the contrary, all can be engaged on equal terms, without one being preferred over the other or without all of them being suppressed and therefore de facto world view secularisation taking place.