The U.S. Constitution explicitly calls for a separation of church and state, yet so often, this is ignored.
I work for a contractor for a branch of the US Army. Many times, as I was leaving work for the day, a kiosk was advertising a "Bible Study Group". The Founding Fathers did not want this sort of co-mingling for many reasons:
1) They had seen what had happened with European states that had a State Religion, and did not want the same thing to happen in the US.
2) They knew that if one religious group were favored, other groups may feel disenfranchised. This country is one of many religions. The kiosk display above clearly favors Christianity, potentially disenfranchising Jews, Muslims, Buddhists, Taoists, Confucianists, and Atheists.