Because they understand RIGHTS ARE ENDOWED THE CREATOR -- the God of the Bible. And, because they openly based criminal law on the Bible:
"Christianity is part of the common law"
[Sources: James Wilson, Course of Lectures [vol 3, p.122]; and quoted in Updegraph v. The Commonwealth, 11 Serg, & R. 393, 403 (1824).
"The Bible was America's basic textbook in all fields."
[Noah Webster. Our Christian Heritage p.5]
You're, apparently, trying to rewrite history just as liberals are attempting to do.
If all it took was Christianity to make a great nation explain to me the dark ages and the failure of the catholic church the UK, France etc...they were after all, all Christian nations where they not? and futher explain to me why the founders thought it was a must to include freedom of religion when forming this nation?
(hint Because they were running from Countries that used Religion as a means for despot rule (i.e Church of england/Vatican))