Expert Answers
rrteacher eNotes educator| Certified Educator

People could be atheists for many different reasons. One major reason might be that they were raised that way, or at least raised in a family where religious belief was not prioritized. Religion is one of many aspects of a person's worldview that are often strongly influenced by one's family, so lack of religion can certainly be the result of an irreligious upbringing. But it is also true that many people raised within, or at least exposed to, religion become atheists. They might become that way for many different reasons. One writer in the 1930s, after facing persecution in India for his lack of religious belief, said that he rejected religious belief because it was not consistent with his ideas about human dignity, and ultimately he found appealing to God less than useful:

I don’t think that by strengthening my belief in God and by offering prayers to Him every day (this I consider to be the most degraded act on the part of man), I can bring improvement in my situation, nor can I further deteriorate it.

Others, like the famous nineteenth-century English author Percy Bysshe Shelley, reject religious belief because they feel it is contrary to reason. Shelley, in an essay entitled "The Necessity of Atheism," wrote that the the mind (perhaps more accurately his mind) "cannot believe the existence of a God." Still others may have become persuaded that no deity exists. This might be because they take a skeptical stance that requires evidence in support of their beliefs, or it might be because some event (the death of a loved one, perhaps) has convinced them that there is no spiritual entity in the universe. 

Finally, many people would respond to this question by saying that they simply don't believe in a god. One recent writer has said that "[w]hen queried on the God question, I simply say, 'I don't believe in God.'" Authentic belief, most would agree, cannot be coerced, either externally or internally--one cannot force him or herself to believe. So some people simply do not believe in God. They may have no easily articulated reasons for their lack of belief; they just don't believe. Still more people would probably classify themselves as agnostics, meaning, basically, that they don't know whether or not, or in what form, a god exists.