Participation in Christianity in the United States is wildly over-reported.

During the Republican primaries pollsters were baffled by then candidate Trump's showing among "evangelicals". A pollster in South Carolina did something virtually no other poll did. Add questions about church attendance. Candidate Trump was through the roof on people who described themselves as evangelicals yet were Christmas/Easter Christians at most. Ted Cruz on the other hand was most supported by those evangelicals attending church regularly. Cruz finished third despite the support of regular church attenders.

The United States has entered what I can only describe as being "a weird place".

You have a significant segment of the population that clings to the labels of religion but reject the orthodox tenets of the faith.

There is a peculiar American jihadist movement that is not unlike the Islamic jihad movement in rejecting the orthodoxy.

Then you have an "enlightenment" movement that will depart from orthodoxy to use the "guidelines" of faith within the context of modern culture (think some of the most Westernized Muslim nations and progressive Christian denominations).

Then you have those who hold to the orthodoxy of their faith but see the secular government as something you might participate in while keeping your faith personal.

Many of the Founders fell in this category. Their distrust of political interference with church (the state declaring rituals and approving ministers/priests) was equally matched by the experience of their forebears when church used the government as a tool of religion. Mandatory baptisms, purges and oppression of churches that were not in power, including the stripping of civil rights from adherents of an out-of-favor church (history of Ireland as a UK colony for example).

Then of course the "nones"

The Founders didn't really have "nones" because science had no explanation for the existence of people or the planet. The nones of that era were generally called deists. A builder god created everything and then either died, moved on to the next project, or just sits back idly watching.