Take for example, someone who was not born in an atheist home, or a religious home, free from even hearing about religion and coincidentally he made up none of his own. Then when this man hit 21 years of age he was exposed to our world of countless religions. This man has no connection to any religion in particular and he is asked to pick one or none of them. He would notice that they are all mutually exclusive, that is no two religions can be right. He would also not be able to determine which had any validity. All he could do was read the old religious scriptures, which unfortunately almost every religion has. You could say that he could judge validity based on the most popular religion, however would you base it on people alive or people dead? Really the most popular religion of all time would not necessarily be around today, though the consensus today would be that it was ridiculous and unbelievable. This being the case, it is illogical to be religious. If someone who has never been exposed to religion comes in when he is 21 (so his brain has developed enough to make a conscious decision) and cannot make a rational decision which religion is right, that means there is not enough logic behind religion to make it a rational decision.
You might say, well if it is illogical then why are there so many people making this decision? My response to that is that there are a large number of causes. First, and in my opinion the most formidable, is the practice of childhood indoctrination. Children are extremely impressionable and when they are indoctrinated in religious beliefs it is nearly impossible to convince them otherwise later in life. This is one of the key agents responsible for perpetuating religious belief. Another reason is that religions are attractive. Many effectively say, if you’re on our side, you get eternal bliss but if you aren’t you get eternal torture. If nothing else this promotes choosing a religious belief over no religious belief. The fact is that religion is not perpetuated by logic, it is perpetuated by childhood indoctrination, rewards, and punishments.