At the time of the Revolution religious freedom was not guaranteed by any government then existent. Most of the colonies had officially established religions and citizens were expected to attend that church and pay taxes supporting that church.
Before the U.S. Constitution was drafted, future presidents Thomas Jefferson and James Madison worked to free Virginia from its official state church, the Anglican Church. Jefferson wrote the bill and Madison pushed its passage as a member of the state legislature.
Shortly after the bills passage Madison went to Philadelphia and became the primary author of the U.S Constitution and the First Amendment. During the lengthy debates between delegates to the constitutional convention several attempts were made to include statements claiming the United States to be a Christian nation but they were soundly rebuffed each time.
There only two mentions of religion in our constitution, both in the negative. Article VI prohibits a religious test in order to hold public office and the First Amendment bars laws “respecting an establishment of religion or prohibiting the free exercise thereof”.