It is a nation without an official religion. That’s precisely why it’s home to the (unfortunately, in my opinion) least secular society in the free world. The absence of an official religion is why America became a magnet for those members of European sects that actually believed in their theology, as well as becoming a laboratory for new extra-strength forms of (and perhaps variants on) Christian theology, such as Assemblies of God, Church of LDS, Missouri Synod Lutheranism, KJV-only Christians, etc.
It’s about in-group vs. out-group, and it’s also a numbers game. Today, to come up with a Christian majority, you have to include a lot of people who would have been classified as non-Christian in previous generations, including a whole lotta Cafeteria Catholics, Mainline Protestants, etc. But the people most loudly proclaiming the Christian majority are the ones who in the pulpits of their own places of worship most narrowly define what it means to be a true believer (and therefore saved, according to their beliefs).