It's a simple matter of historical record, but revisionists have worked hard to distort the truth about American history.
That said, to understand the "Christianity" of our individual Founding Fathers we need to compare what they wrote with the Bible's definition of what a Christian is. In doing so we find that not all of our Founding Fathers were devout Christians, but all did have an understanding of the fact that Bible principles are for the good of mankind. They recognized that Christian values promoted stable families and communities, and were for the well-being of all peoples.
The Christian values of men like Abraham Lincoln broke the practice of slavery in our country, for instance, (the same is true of men like Wilberforce in England), as well as established a legal system that had the potential to continuing developing for the benefit of this country's citizens.
Sadly, instead of continuing to develop our country from a foundation of Christian values, this nation has steadily moved away from those values.
There's more to it all, obviously, and I'm veering from your main topic, so I'll quit here, :) but it will be interesting to follow the answers to your Q.