Washington, Madison, and Monroe were active in their Episcopal congregations. One of the signers of the Declaration was the Reverend Doctor John Witherspoon, an influential minister in the Presbyterian church, president of Princeton university (then the College of New Jersey), and descendant of John Knox. John Adams attended United First Parish Church in Braintree, where he and his son John Quincy are buried.
The country wasn’t as nearly Christian as most moderns think. There was quite a bit of skepticism about establishment clerics among the country folk, and a lot of negative humor thrown their way. Read something like James Fenimore Cooper’s “The Spy.” He portrayed a lot the commoners as religious cynics.
I do not believe I would term Franklin a lifelong "agnostic." He was baptised and grew up a Congregationalist and one of his most famous acts was to call for a day of prayer the day the Constitutional drafting was most at peril, June 28, 1787. The text of the Franklin's motion reads:
" I therefore beg leave to move, That henceforth Prayers, imploring the Assistance of Heaven and its Blessing on our Deliberations, be held in this Assembly every morning before we proceed to Business, and that one or more of the Clergy of this city be requested to officiate in that Service. "
As for Jefferson's Deistism, such religious philosophy does not make him less a Christian.