You don't know what you are talking about. The founding fathers were certainly not all secular. I highly doubt that ANY were atheists, precisely because of the founding principles.
There is a common misconception, often presented by seuclars, that the founding fathers were "Deists" or subscribed to no particular religious belief... and the evidence is the lack of evidence to show them connected with any particular church or religious denomination. However, we have to remember a few important things here, we are talking about a time when "media" was fairly non-existent in America, and public perceptions were molded and shaped mostly by direct speeches witnessed first-hand, or reported months later in the press. It wasn't very difficult for the Founding Fathers to keep their private life private, they simply didn't talk about their religious convictions. Now, it's very important, when you are trying to establish a religiously free society, that you don't appear to be aligned or affiliated with one particular belief, because then it becomes quite a conflict of interest. Therefore, much of the details covering the fathers and their personal religious beliefs, is simply missing from the record, by design. They were compelled to not try and sway or influence the public by pronouncing their own beliefs, because of the positions they held. It was far more important to establish a government where religious freedom prevailed, without the undertones of religious manipulation or entanglement. They could never have done that, if they were running around 'preaching' their religious views. So the founding fathers mostly remained private on what they personally believed, and modern seculars now wish to take the 'lack of evidence' and turn it into something it's not.