I always just point to the Middle East as a first-hand example of the problems societies have to face when religion and government overlap. Then I remind them of the Dugger (sp?) family and ask them if that's really how they want everyone to live. Then I generally walk away, because in my experience, people who need it pointed out are the ones least likely to get it anyway.
I'm going to hit this hard in my poli-sci debate tomorrow.
Those fanatical believers, be they Muslim or Christian, who fervently yearn for a theocracy, doing their god's will and having heaven on earth, sooner or later get bit in the arse by the law of untended consequence when they end up with a religious tyrant. The trouble is that so many of them will still continue to religiously obey their tyrant.
This country would be no different as a Christian theocracy as the Muslim ones that so many Christians purport to hate.