Weren't our Founding Fathers Christians?

No.

A misconception exists in this country that the founding fathers of the United States were Christians. Although some were, most were not. The major figures in our history were deists.

The great leaders of this country, Thomas Jefferson, Ben Franklin, James Madison, John Adams, and Thomas Paine, were British subjects who were highly educated. As such, these men would have been indoctrinated in philosophical thought of the eighteenth century enlightenment.
Originating from Hobbes and Locke, Natural Rights Philosophy espoused the theory that one can come into contact with the truth of inalienable rights by means of reason. In the state of nature, persons congregated together to form government in order to protect these rights. These individuals initiated a contract between ruler and ruled. Any reference to God in The Declaration of Independence was based on a deistic notion of God as absentee landlord.