You cannot replace God before you deny it. Otherwise it's an oxymoron.
Replacing one god for another is not atheism. Either way it doesnt make atheism the basis of socialism so much as the elimination of private property rights is the basis for socialism.
TBH I think most Christian's and Muslims do not consider their God a highest authority. It seems most people just do it because everyone else is doing it. Only extremists are believers
You should begin to read both the Bible and the Qur'an if you want to have a credible opinion on them both and have meaningful discussion. It's kinda of obvious you haven't.