It was founded as, and was assumed by its founders always to remain, a nation comprised overwhelmingly of Christians who would be influenced by their faith in their politics.
However:
* The things that the founders actually DID with Christianity were, by the standards of European politics, and English law, RADICALLY secularizing. No established Church, and zero meaningful subsidy for churches generally. (Religion is FAR more subsidized in 2014 than it was in 1789 because of the deductibility of contributions from taxation — neither income taxes nor deductions therefrom existed.) No prohibition of Jews or Catholics from public office, jury service, certain professions, etc.
* Many of the founders were not at all orthodox Christians — many plainly did not take the Bible seriously at all as divine Word, and many who did were highly heterodox, especially with regard to the divinity of Christ. It should not be inferred that they intended their descendants to be bound by law to beliefs or practices they themselves did not hold to.
* The things we think about as distinctively “Christian” in politics in 2014 were simply not issues in the late 1700s. I don’t think that Christian super-majority character of the nation was expected to have any relevance to illegitimacy, abortion, divorce or homosexuality, or the relative authority of husbands and wives over household and family. Even if we were “founded as Christian nation” is doesn’t actually mean anything in terms of what policies we should endorse or not.