It's no wonder that freedom of religion was on the founding fathers' minds. They were trying to craft a country out of 13 states which did not have uniform views on the subject, and whose views on the subject dated back to the founders of those states. Furthermore, as colonies, originally they had "Established Religions".
In Massachusetts Bay Colony and Connecticut, the "Established Religion" was a very strict Jonathon Edwards form of Congregationalism, sometimes called Puritanism. Plymouth Colony was similar, but not as strict, at least at the beginning. However, both colonies also had the Congregational hallmark of believing in local, democratic control of the single churches. These beliefs sometimes became at odds with one another. And in fact, the "mother church" at Plymouth voted to become Unitarian later. (Unitarianism was founded after the Constitution was written, but not by much).
In Massachusetts, at least at the beginning of the colony, it was forbidden to celebrate Christmas, which was regarded as a pagan festival, and church attendance at the Established Church was mandatory.
It was typical in Massachusetts to tar and feather & otherwise persecute Quakers and other dissenters from the established church. Needless to say, Quakers founded a different state, Pennsylvania. And Roger Williams founded Rhode Island on a basis of religious tolerance, which is why the first Jewish synagogue in the US was founded in Newport, RI. Ethan Allen and the founders of Vermont were Deists and the old church in Burlington is a Unitarian church. Delaware was founded by Catholics, who were persecuted in some other areas of the country. And, AFAIR, Virginia was founded by Episcopalians.
If you wanted to join all these different interests into one country, you had to start by assuring every colony/state that they would not be forced to practice someone else's religion by the new government. And because you had Deists and the Rhode Islanders involved, you had to be even more tolerant to draw in RI and VT (and in fact, VT did not ratify the Constitution until AFTER the Bill of Rights was added. VT had its own Constitution, which banned slavery, prior to the US Constitution.
And, at the time, there were organized Indian tribes, particularly the Seven nations in upstate NY and the Cherokees in NC, who were not Christians at all. It was not until the 1800's that there was an effort to expel all Indians from the eastern US who were not Christians. At the time of the founding, there would have been a mindfulness of their Indian allies. (Many of the Indians were on the British side, however) In CT, the Mohegans fought for the British and the Pequots fought for the Americans. Not surprising, since they were traditional enemies of each other).
In Europe, of course, different sects of Christians had been murdering one another for years. Many of the colonists had fled Europe to get away from that sort of stuff.
It's not surprising that the Constitution was written with Freedom of Religion written in it.
And BTW, the pledge of allegiance did not have under God added to it until 1954, at the instigation of the Knights of Columbus, the Sons & Daughters of the Revolution, & the Prayer Breakfast group in Washington, DC.
http://en.wikipedia.org/wiki/Pledge_of_Allegiance
I remember this well, since as a child, I first learned it without the words "under God" and then had to learn to insert them later.
