The assumption that America was founded on Christian principles is ludicrous. The First Amendment proves this. We should all be thankful America hasn't fallen under the control of any religion, lest we return to burning witches, slave ownership, stoning non-virgins and sabbath workers, etc.
Our nation is bad enough with its unjust persecution of gay people based on religion. To insist we abide by doctrinal law is to condone murderous acts committed under Sharia Law in Islamic nations. I doubt any Christian would want their 15-year-old son tossed off a building because he was accused of being gay. This recently happened in Syria.
How do we decide what doctrinal laws are to be taken serious or rejected as being barbaric? How and where does one draw the line between magical, mythical impossibilities and just plain common sense to live a moral life, then decide what is or isn't to be taken to legislature? As an atheist, I've never needed a guide book full of magic and barbaric dictates to know how to be nice to my fellow man.
Religion is the last thing any healthy society should infuse into secular law as ISIS, al Qaeda and Hezbollah have demonstrated. Never mind what the Catholic Church did to anyone who impeded their dominance of society. When anyone claims that our "American heritage is dead" and accuses rational thinkers of being "elitists" and "fascists" only because we don't favor their chosen, divisive god's cherry-picked mandates, they need to look into a mirror.
If one feels the need to give over control of their lives to an imaginary dictator, that's fine. All I ask is that they keep their religion where it belongs -- within their imagination and out of the government and out of the rest of the American public's lives.