60%? Serious citation needed. The largest Christian population in Iran are Armenians. There are far fewer than 1 million Armenians in Iran. So unless you have evidence for the claim that there are 50+ million atheists in Iran, the number just defies belief.

I would be shocked if there were 50 million atheists in America. Maybe if you included people who are spiritual but do not believe directly in a god. Maybe I could accept it then, but at that point, you are stretching the definition of 'atheist' to its breaking point.