The USA was founded as and still is a Christian nation that has a secular government. I don't know why this is so hard for so many people to accept. It seems that the anti-theists and irreligious people of today feel threatened by this obvious fact. And I can understand it to a point - I know that there are people who wrap themselves in a flag of religion (Christianity) and try to use the fact that we're a Christian nation to bully non-Christians into conceding to "Christian" laws, displays, etc.
But nobody should feel threatened even if there are people trying to use "Christianity" to bully them, because the fact is that the government is secular and this is clear in the Constitution. But make no mistake, the vast majority of the founding fathers were Christians and the nation remains a de facto Christian nation. So why do we have a secular government? Because this was the CHRISTIAN decision made by the Christians who founded the nation. The idea of separation of Church and State was wildly popular among European Protestants because they saw this as the only way to have the freedom to openly and freely practice their denominations' form of Christianity.
Yes, there were a few deists - literally just a few. And yes, the deists were on board with the idea of separation of Church and State. But again, the majority of the funding fathers were Christians, the concepts of freedom and free will in our founding documents are rooted in the Christian concept that God gives ALL men a free will in choosing to accept or reject Him. It is a Christian concept to extend that free will to others and not coerce them into abiding by Christian laws or rules.
Even so, many of the laws for the large part of the US's history were Christian in nature. Blue laws, for example - some are on the books to this day. Paramus, NJ is the 2nd largest retail center in the US (after Fifth Ave. in NYC and ahead of Rodeo Drive in Beverly Hills). However, Paramus' huge retail operations are CLOSED on Sundays. Why? Because of a Blue Law that is still on the books.
Also the culture in this nation is undeniably heavily influenced by Christianity over any other religion. Judaism is a distant second in the influence in our nation. Look at our cities and look at how many organizations and businesses have a Christianity-influenced name. Cities like Los Angeles, St. Louis, San Francisco, Corpus Christi, Santa Barbara, St. Paul, etc. etc. and companies like St. Joseph's aspirin or the many hospitals such as St. Luke's or Baptist General or St. Vincent's...
We are clearly a "Christian" nation. It is de facto and it is nominal but Christianity is enmeshed into our culture. Two nationally observed holidays are exclusively Christian - Christmas and Good Friday (since Easter is on Sunday Good Friday is observed as a day for businesses to close and people to have off). And although it's not explicit, it's de facto that Thanksgiving is rooted in Christianity (giving thanks to Whom? God, of course).
So yes, the nation was founded by Christians and was intended to be a nation where Christianity could be openly and safely practiced and grow and thrive... However, in their wisdom, the founding fathers created the government to be secular. Despite that, however, this nation has been a majority of Christians (nominally, anyway) and remains such to this day.
The USA was founded and remains a Christian nation, albeit one with a secular government that protects the rights of non-Christians equally with Christians.