text: I suppose it comes from our "Puritan" roots.  The first pilgrims to come here (European, mainly English anyway) came with the intent of setting up religious communities.  That is where we get the idea of freedom of religion- because they were escaping persecution in Europe- but in reality their rules were every bit as (if not more so) religiously tyrannical than where they came from.  I guess that rather than dying out through the years, the idea that "our way is the only way and you can go to hell if you don't follow us" has continued and in some ways grown.