Ian,
Most people had a religious affiliation before they became atheists. Many of them were Christians but not all of them. There are atheists from all religious groups. For some people it is very difficult to rid themselves of oppressive religious dogma, but those that do never seem to want to go back.
Yes, the most repressive religions frown on asking questions. People that learn to ask questions frequently become agnostics or atheists. When people become more educated they often reject dogma that was pumped into their heads all their lives. Countries that have a highly educated population usually have a high number of atheists and agnostics. It appears that better education creates atheists.
People that were never indoctrinated are in the minority in America, but studies have shown that countries with a modern education system and low illiteracy have a large number of atheists and agnostics. These countries have far less violent crime and much better social system (they care more about the people.) People that are allowed to ask questions (and get honest answers) have less trouble distinguishing between what is true and what is a prevarication.
It appears that discouraging curiosity and dumbing down the population is a good thing for those spreading superstition and religious dogma.
