What does this quote mean?

It means that "thinkers" and people who do a lot of pondering generally arrive at the conclusion of atheism. This is because, "thinking men" are typically INTP personality types, which means they make decisions based exclusively on logic and reason. To them, and myself, it is only logical to ume that humans were not created with magic and are not watched at all times by a bearded man. Thinking men will tend to trust science over religion.