There is abundant documentation from this period. Only if you carefully cherry-pick the evidence can you hold the theory that all or most of the Founders were aggressively fundamentalist Christians of today’s type.
They were also not secularists by today’s standards, as others try to claim.
At the time, aggressive religiosity was considered to be in poor taste, more than anything else. Christianity was accepted as a background to society, for the most part it was not something people got excited about, either in favor or against. Most of the Founders were Christians, although many were not terribly devout, and some of the leading Founders were not Christians at all in any traditional sense.
America became much more religious in the Second Great Awakening from 1800 to 1830. The first Great Awakening was roughly from 1730 to 1745, so the Revolution occurred during a lull in religious activity in America.