This might surprise you, but the first Anti-theists in the USA were Protestant religious extremists, who didn't consider their Christian religion a religion, they considered it 'spiritualism', and used it as a weapon against Indigenous peoples, Catholics, Muslims, and more who they said were evil.

I'm not that surprised, that kind of rhetoric still happens all the time. "Spiritual not religious" is basically a dog whistle for "my religion is better than yours morally".

These Protestants believed that their beliefs were the opposite of Religion, because they weren't 'organized', and that everyone who opposed their belief system were evil scammers and/or wielding the power of demons, and not to be trusted. People in that faith often got funneled into the KKK.