The job of a king is [to] reign, which means that by simply existing and being King, he prevents the negative sum struggle for power from destroying the wealth of the Kingdom and possibly getting lots of people killed. His job is to deny political power to anyone and everyone that wants it.I mostly posted this because that first para in particular is very good, almost aphoristic. But the whole piece is a good read.
The job of a king as head of the official Church is to prevent a negative sum theocratic struggle for power, to prevent people from advancing their political ambitions by being holier than thou. By preventing a theocratic struggle for religious authority, he prevents religion from being perverted into an instrument of power, and thus prevents morals from being corrupted by those who most loudly proclaim their greater holiness.
As Jim points out, God is dead, so the potential for theocracy as such (that is, involving belief in God) is pretty much null. (In the West, anyway.) On the other hand, the potential for atheocracy is as high as ever. So a secure state must either be able to ignore belief entirely, which assumes very strong state security, or it must occupy the job and prevent atheocratic capture. This strikes me as something that will happen without effort for a neocameral regime, but I'll have to think on that.