The Library of Congress has an entire online exhibit refuting your claim.
You can spin it any way you want - take a look around - turn on a radio, or television, or wander into a movie theater or book store and tell me again how Christian this nation is....
The exhibit you link, a retrospective on the influence of religion in the history of this country, offers nothing to change the fact that this nation is not a theocracy. And that is a GOOD thing. Even as a Christian I do not want my religious leaders in charge of whole of society, that is not their calling, and neither do I want my political and business leaders in charge of my church.
Seriously, can you look around and honestly say this is a Christian nation, or even a nation of Believers? Easter Christmas self proclaimed I once went to a Sunday School class when I was twelve and I say I believe in Jesus ‘Christians’ who neither live as though they believe nor do they promote faith and virtue do not make this nation “Christian”.