Really? Because (imho) fear of damnation is the flip-side to the promise of salvation. People are told they need to convert or be "born again" in order to be saved. This is the very means through Christianity (as part of a colonial policy of conquest and genocide) converted one-third of the world.
And yes, Christianity has lost a lot of followers because its traditions and ideology are eroding, and have been for centuries. But that's not the reason for tribalism in America. The reason is the struggle that exists between progressives and regressives, and the latter use religion as a weapon.
They not only claim America is a "Christian" nation (contrary to what the Founding Fathers made blatantly clear), they claim evolution is bunk, homosexuals are "grooming children," and science and literacy are somehow "elitist." They want to keep the people ignorant and fearful so they are easier to exploit.