"The United States is a Christian-founded nation"

[A Theist]

Our nation is culturally Christian.

Yes, the people are.

Our government is not.

At least not at the Federal level. It was a different story at the State and local levels though. Some States Constitutions went so far as to require that candidates seeking political office had to believe in God and be a Protestant Christian. Like these for example...

Delaware; Article 22 (1776) "Every person who shall be chosen a member of either house, or appointed to any office or place of trust...shall...also make and subscribe the following declaration, to whit:

'I,_____, do profess faith in God the Father, and in Jesus Christ His only Son, and in the Holy Ghost, one God, blessed for evermore; and I do acknowledge the holy scriptures of the Old and New Testament to be given by divine inspiration'"

Georgia; Article VI (1777) "The representatives shall be chosen out of the residents in each county,...and they shall be of the Protestant religion..."

Massachusetts; First Part, Article II (1780) "The governor shall be chosen annually; and no person shall be eligible to this office, unless...he shall declare himself to be of the Christian religion."

Massachusetts; Chapter VI, Article I (1780) "[All persons elected to State office or to the Legislature must] make and subscribe the following declaration, viz.

'I,_____, do declare, that I believe the Christian religion, and have firm persuasion of its truth...'"

http://www.constitution.org/primarysources/state.html

There were also many local laws "Blue Laws" that enforced "Sabbath Keeping", some of which remain to this day....

http://legal-dictionary.thefreedictionary.com/Blue+Laws