If you read the First Amendment to the U.S. Constitution, Congress is prevented from making any laws respecting the establishment of religion or prohibiting the free exercise of religion.
In other words, the government cannot make a law establishing an official religion, nor can it keep people from practicing their religious faith.
We are not a Christian national. We are a national free to exercise the religion of our choice, be it Christian or otherwise.
As a Christian, I exercise my choice to worship as a Christian, but I respect the rights of those who do not believe as I do.