Walking through the local college campus, one would be confronted on any given day around the student center by evangelists preaching the evils of the secular pursuit of knowledge. On one particular day, as one such individual took to shouting at me that I was going to Hell for "fornicating and drinking", I stopped and asked him a question. The conversation went as follows:
Me: Do you believe in faith?
Me: Do you believe in revelation?
Me: Well, God spoke to me last night - I have faith it was Him. God told me if I was walking down the street and some asshole told me I was going to Hell to ignore him.
He jibber-jabbered some sort of response, but I was already walking off and had no further interest in hearing him.
Perhaps my tactic was a bit crude, but it illustrates an important point: faith and/or revelation can not be the basis of policy in a pluralistic society because it is not open to debate, there is no means by which to resolve dispute. The evangelist could not dispute my revelation because faith is by definition beyond dispute - it is belief despite evidence or inspite of contrary evidence. This is the reason government must be secular, it creates a forum where those of differing beliefs can meet on equal grounds and discuss the merits of policy; it creates a marketplace of ideas.
A reason governments based in orthodoxy (of any sort) tend towards authoritarianism is that a hegemony of ideas can only be maintained by suppressing contrary ones.
To see this position argued a bit more persuasively, read A.C. Grayling's brief essay "The Secular and the Sacred."