People point to certain letters of the founders who were clearly Christian and say, "see?" To which I reply, "see what?" It's what is in the founding documents that matters, and they have nothing in them that would indicate the founding of a Christian nation. Go ahead and look for yourself; you will look in vain. That we are and have always been a nation deeply steeped in a predominantly Christian culture is not to be doubted; that we have never had a "Christian politics" is equally unassailable.
Supposing for the sake of argument that all of our founders were Christians (which was most assuredly not the case), the question that begs to be asked is why they did not avail themselves of a singular opportunity to found in this "new world" a full-blown Christian theocracy from the ground up?
There is an answer to this question, but it is one that those with no grasp of European history have trouble fathoming. A crucial majority of the founders, along with the Christians among them, had fresh in their minds the vast amounts of blood that had long soaked the soil of the old world as Catholics and Protestants sorted out who had the superior interpretation of the same holy book. The vast majority of the founders were from the British Isles, which had also been infected by the disease of religious intolerance. No doubt many of them had ancestors who suffered persecution at the hands of the state, depending on the religion of the person seated on the English throne, be they Protestant or Catholic. They wanted to inoculate these shores from the same vile disease, and so they eventually produced the brilliant first amendment to the constitution.
To my mind the term "genius" is far too often applied to the founders, given the significant number of very good ideas they took from the Enlightenment thinkers of England and France, as well as some of the very best ideas of antiquity. But here, with the first amendment, they struck a Solomonesque balance between freedom to worship according to one's conscience, and freedom from the imposition of religious orthodoxy at the hands of the state, thereby rightfully earning this accolade and securing their legacy.
But sadly, it is this legacy that is currently under renewed attack by self-described piests and "patriots" who remain willfully and stubbornly ignorant of this, the founders' greatest achievement.