I am an atheist who was raised Christian in a small town in a red state, but spent much time trying to understand religion. And truly no one evangelizes more than atheists who were formally Christian. The level of vitriol and lack of curiosity and nuance is literally the same for both groups.
Anyway if your only understanding of religion is Christianity, and you are fine with ignorance and completely lack any curiosity about how other faiths actually work culturally, it becomes obvious why Christian atheists refuse to believe someone can be atheist and also Jewish or Buddhist etc.