The phrase "Founding Fathers" is a proper noun. It refers to a very specific group of people, the 55 delegates to the Constitutional Convention. Yes, there were other important players, like Jefferson, whose thinking deeply influenced the shape of our nation and who were not in attendance, but the 55 Fathers make up the core.
The denominational affiliation of these men is a matter of public record. Among the delegates were 28 Episcopalians, 8 Presbyterians, 7 Congregationalists, 2 Lutherans, 2 Dutch Reformed, 2 Methodists, 2 Roman Catholics, 1 unknown and only 3 deists--Williamson, Wilson and Franklin, this at a time when church membership entailed a sworn public confession of biblical faith.(John Eidsmoe, Christianity and the Constitution, (Grand Rapids: Baker, 1987), p. 43.)
This is a very revealing tally. It means that the members of the Constitutional Convention, the most influential group of men shaping the political foundations of our nation, were almost all Christians, 51 of 55--a full 93%. Indeed, most were Calvinists (the Presbyterians and the Dutch Reformed), considered by some to be the most extreme and dogmatic form of Christianity.