I have, for a while now, been attempting to communicate to very well-meaning folks that there is not, and has never been a truly Christian nation; and that this statement includes America. I mean to tackle this at length at a later date, but for now here are a couple quotes from Rodney Stark's book, "The Triumph of Christianity" (HarperCollins, 2011). Emphasis is added.
"In 1776, on the eve of the Revolutionary War, only about 17 percent of those living in one of the thirteen colonies actually belonged to a religious congregation; hence more people were probably drinking in the taverns on Saturday night than turned up in church on Sunday morning. As for this being an 'era of Puritanism,' from 1761 through 1800, a third (33.7) of all first births in New England occurred less than nine months after marriage, and therefore single women in Colonial New England were more likely to engage in premarital sex than to attend church." (353)
"The very low level of religious participation that existed in the thirteen colonies merely reflected that the settlers had brought with them the low level that prevailed in Europe. Keep in mind that few of the colonists were members of intense sects who had come to establish Zion in America - Puritans did not even make up the majority of persons aboard the Mayflower. That the Puritans ruled Massachusetts, imposing their morality into law, has tended to mask the fact that, even in Massachusetts most colonists did not belong to a church congregation - only 22 percent did belong." (354)
Hopefully this gives you some food for thought. This country never has been the massively Christian nation that many people seem convinced of. It is easy to look back and romanticize the past. This is not a wise thing to do (Ecclesiastes 7:10). We should be thankful for the good in our history, without feeling the need to cover things we might find less pretty.