No it isn't... But the United States are a Christian nation in a way European countries aren't any more... Just compare the Churches here and in Europe on a sunday...
That first sentence of the Bill of Rights characterizes this nation as not of ANY particular religion. Our society is constantly changing as far as percentages of religious belief and practice which is what the founders had in mind - NOT making this a nation of one particular religious belief. The particular religious belief of any one of the founders is irrelevant to this point. We are a FREE nation of all religious beliefs.