But here's the thing: if what's at issue is the separation of church and state, the founders' religious beliefs are mostly beside the point. If we're trying to figure out the original intent of the First Amendment*, what matters isn't whether they were religious or not, but whether they believed in separating religion and government. Which most of them did, whether they were orthodox Christians or not.
These days, it's easy to get the idea that church/state separation is a secularist idea, because many of its most vocal advocates today are secular. People think that if you're a conservative Christian, you must oppose separation. But that's not always true, and it certainly wasn't true in the early history of our country. You can be a Christian and still support separation of church and state. In fact, Christians are the ones who first fought for it.
In the colonial era, many of the the states had established churches, and actively persecuted dissenting sects. Quakers were hanged in Puritan New England. Baptists were jailed in Anglican Virginia. Anti-Catholic sentiment was widespread, to the extent that George Washington had to forbid his troops from burning effigies of the Pope.
Not surprisingly, then, it was minority Christian groups who first wanted the state to stay away from promoting religion--because they knew the official religion would likely persecute them. In Virginia, Baptists teamed up with the Enlightenment rationalists Madison and Jefferson to push the Virginia Statute for Religious Freedom, which would influence the drafting of the First Amendment to the Constitution, as well as the No Religious Test Clause ("no religious test shall ever be required as a qualification to any office or public trust under the United States").
When Jefferson first used his famous and controversial phrase "wall of separation between church and state", he was writing to his allies: Baptists in Danbury, Connecticut. Not only that, but he was echoing words written over 150 years earlier by the religious dissenter Roger Williams, who had founded Rhode Island as a haven for religious freedom. Williams--no stranger to state-sponsored religious persecution himself--had written in 1644 about the dangers of gaps in the "hedge or wall of separation between the garden of the church and the wilderness of the world."
So, while it's an interesting exercise to try to figure out the divergent and complex religious views of the various founders, it's beside the point when it comes to church/state arguments. Then, as now, many of the biggest supporters of separation were devout Christians, who agreed with James Madison when he said:
Who does not see that the same authority which can establish Christianity in exclusion of all other religions may establish, with the same ease, any particular sect of Christians in exclusion of all other sects?Who does not see this? Sadly, a whole bunch of people these days, especially among evangelical Christians. Perhaps they should take a look back at their history; back to the days when they were the ones being officially persecuted, and could still see the wisdom of that wall of separation.
* There are, of course, problems with relying on original intent in interpreting the Constitution. First, there never was one single original intent. The framers and ratifiers didn't all agree on what it meant, or what it should mean. Second, there are good reasons to be cautious about binding ourselves to the views of people who lived 200 years ago. It won't do to disregard their intentions in writing the Constitution, but at the same time, we've moved on from their views on race, sex roles, and many other things, and it's a jolly good thing we have.