When did Church and State become fused together in America?

When did America allow the Church and state to become fused. One of the most powerful key elements now in a presidential candidate is his or her religion. President Bush proves how religious America really is. With so many votes flowing his way because of his religious views and beliefs it goes to show how powerful and present religion is in the United States. Religion is supposed to be kept separate from government decision making and policy formation. Religion is almost always present in debates between candidates. But why, in a progressive country like this one why should religion play a role on how a president runs and directs a country.