As with a lot of political subjects, this one in particular is a lot more complex than a bumper sticker quote.
We can see, as could our founding fathers, from history what the effects are of Christians taking over the government and attempting to run it as a theorocy. The movement, probably most like the current climate was the Calvinist movement in the 16th century.
Check out : http://www.wsu.edu/~dee/REFORM/CALVIN.HTM
This is a good history lesson, but you have to keep going for several pages of topics.
The effect of calvinism is found on about the 3rd page at: http://www.wsu.edu/~dee/REFORM/WARS.HTM
If you read these, you can see the danger of Calvinism if left to continue.
Our country was founded by 2 separate groups of people-
> those who wanted to rid themselves of the religion of the state , and purify it back to what they thoought it should be- Called themselves the Puritians. These were the religeous people we hear so much about. It is from the Puritians we get so much of our "thou shall not's" around here.
> Those who were totally secular and came to this country to get rich (after all the Spanish had found lots of gold), or because their life in England and Europe was so miserable, just to start over.
Our founding fathers for the most part were Free Masons, and therefore considered all men to be equal. They lived in the time of the enlightnment, and thought that a country should be governed by secular laws, while letting people worship as they saw fit. This is not to say that they were non-religeous, but to say that they thought religeon, most especilly state sponsored religion, should not have a place in this government.
As better put by those before me, we are therefore a nation made up primarly of Christians, but lest we forget the past experience of Calvinism, and the Religeous Wars it brought, We should not be governed as a theorocy.