Atheism is not an inborn trait. It establishes itself with the passage of time. It grows gradually as well as ones intellect grows.
I have not seen any baby atheist. Baby religious can be seen because his elders start his education in early days and he starts uttering religious concepts with his stuttering tongue, but what should a baby with an atheist background utter?so it’s difficult to see some baby atheist. It happens when a man is grown up.
A man becomes an atheist by many reasons. One of them is the same like of a theist, that he inherits it. His culture and locality is of atheists. He finds himself in such an atmosphere, where no one bothers Deity or some supreme being. They do not discuss God and beliefs like, life after death, heaven or hell, angels, Day of Judgment, Hereafter or Thereafter and many more, so his ears are not tuned with these kinds of sounds and concepts.
The second reason is somewhat different from the first one and that is, he is born with some religious culture, as long as he steps up the stairs of his age, knowledge, learning, studies, intelligence and intellect, he becomes skeptic. Here is an important point must not be missed that he is born with intellect, he is not a duffer, he is sensitive and somewhat logical. He measures beliefs on the litmus test of his arguments. To be brief, he is an intelligent person who can not endure such beliefs for which he has not arguments.
So what happens? Ultimately his skepticism lingers him to the valley of rationalization, the desert of plus minus and infinity, the way where logic and intelligence are his torch bearers, where baseless concepts are not entertained.
Here is the stage when his I.Q. and E.Q. do not match his search, but as long as he travels through this dense rain forest of rationalism he comes to know that these are mere beliefs and are man made concepts, he smiles over the innocence of staunch followers that what is the thing they are running after?
On this stage he may become agnostic and some time it happens that an atheist was an agnostic first and then he denied everything and became atheist but it is not necessary fact, it depends upon his moral courage that either he is able to deny everything at once or not? If he still feels thirst for more he does not declare himself an atheist, but when he thinks that his search is over and nothing can make him convinced over such ideas, he becomes declared atheist.
Helplessness, insufficient phenomenon to prove Deity, lesser knowledge of the leaders, lack of research in religions, conservative thoughts of the people and many other things make someone a rebel. The term rebel I am using is in such a context that inwardly may be possible that some atheist still has some soft corner for religion, but apparently he makes a fun of the theists and believers. He wants to see the all at one platform whether it is a sacred lie, but it must be international. Personal game of Dual and fight over schools of thoughts make him annoyed and he feels safe in a condition to deny everything instead of complexity of BELIEFS.