To blame atheism for progressivism is a severe cope, so one can avoid discussing demographics and sex, because gender is not a thing.
Atheism is a factor, I'll acknowledge it.
But the main cause of progressive, liberal politics is the groups that have banded together to undo the "Patriarchy", the dominance of the "white male", legacy of colonialism and more.
It's largely a coalition of weak groups in need of state protections, afraid of societal rules, interdictions and order.
Christianity is actually VERY weak here, and even weaker in general. The people that you see being "religious" online know 1/10 of what I do about their holy books and God, and I am an agnostic. They do not pray. They do not go to church. They live unholy lives, slaves to their hedonism and capitalistic consoooming.
They are LARPers in every way.