Every atheist has his own story about how he became an atheist. No atheist takes only one path; there are often many factors that take one to atheism. These factors are grouped into four distinct categories or "paths to atheism". An atheist may be the result of any combination and degree of the four paths.
This is the most common path taken to atheism. The individual discovers their lack of belief in a god or gods by way of examining the definition of and the beliefs surrounding the existence of a god or gods. The disbelief or lack of belief is found either by concluding that the god cannot exist or that there is no reason to believe that the god exists.
The individual finds that the beliefs of a particular religion does not fit them or has let them down and rebels against it by heading in the opposite direction towards non-belief.
This situation is where atheism is freely or forcibly taught to the individual. It is also possible for an individual to indoctrinate themself.
This is probably the least common path to atheism taken by adults. This is due to the aggressiveness and commonness of religion. When the concept of the existence of a deity is unknown, the individual is an atheist. A newborn baby is an atheist; the idea of a god is something that is taught or discovered.