There’s a famous definition of knowledge, attributed to Plato, that says knowledge is Justified True Belief (1). One of the points of Justified True Belief is that it is not sufficient for an idea to be right, it must be right for the right reason; it must be justified.
If you ask a dozen freethinkers, humanists or atheist to explain why they don’t believe in deities, you will hear a gamut of reasons ranging from their gut instinct telling them so, to Occam’s Razor, to elaborate philosophical arguments that would challenge an Apologist. Rarely will you hear that humans have evolved to have big brains and social instincts that led to the perception of agency. Rarely will you hear reference to W.D. Hamilton’s theory of Kin Selection(2), or E.O.Wilson’s concept of sociobiology (3) or its modern flavor, evolutionary psychology. In short, you will not hear sound scientific reasoning for the freethinker’s ostensible beliefs.
Prior to Darwin’s theory of evolution, anyone’s guess as to the nature of religion and deities would have been as good as any other. Human’s simply had not figured out the process by which they came to be, and the myriad explanations humans had invented were ultimately fictions. The idea of evolution gave us the means with which to understand ourselves. In particular, evolution psychology can now explain why historically we had attributed much to supernatural influence(4). There is now a growing awareness that religion is a psychological and social construct that can be explained scientifically(5), without resorting to gut feeling or unfounded philosophies. We can now be right for the right reasons!