David Fitzpatrick has an interesting summary of arguments over how religious and Christian the Founding Fathers were, focussing on:
"Mr. Barton, who is also the vice chairman of the Texas Republican Party, is a point man in a growing movement to call attention to the open Christianity of America's great leaders and founding documents. The goal is to reverse what many evangelical Christians claim is a secularist revision of history, to defend displays of religion in public life and to make room for God in public school classrooms."
I'm comfortable that they were (mostly, coolly) religious, but it blurs the picture to focus on Washington, Jefferson, Franklin et. al. The best estimates are that the Revolutionary generation was mostly unchurched, with church membership in the 15-25 percent range. See a very interesting book called "The Churching of America" by Roger Finke and Rodney Stark. They say church membership/religious participation grew over the years, reaching a high in the 1960's with reasonable stability since. The reason for the "churching", compared to other developed countries, is the free market competition for members by the various American sects/denominations.
It's also true, IMHO, that we are today a more truly Christian nation in our practices than in 1776. We come closer to practicing the Golden Rule, than at any time in the past. Apply the John Rawls test--forgetting about standard of living, would you prefer to have been born in a time of slavery, patriarchy, etc. or now? I rest my case.