My mom was agnostic, and my dad is still an atheist, and with that background it's obvious that I did not grow up or was raised in church. I considered myself to be an atheist, myself, by every stretch of the word all the way through high school and a few years after. Now through a series of events I have come to grips with reality and realize that Jesus is in fact Lord and that He does exist.
Being raised to believe in god probably is the main reason people believe in something so stupid, but I am sure there are people who were never taught about religion or told religion is not true, and then some idiot comes to you and says "If evolution is true, why are there still apes?" or any stupid argument like that. Anybody who is scientifically illiterate can believe in god.
Yes, you can believe in anything without being raised to. Since the existence of a god(s) of an form cannot directly be proven or dis-proven it's not entirely hard to believe that there may just be something out there. With that said however, religion is a whole different animal. Believing that something exists in the endless reach of space seems easy in comparison to a being that created us for the sole purpose of praising him and condemning us for being impure after being created with an ability to sin. Religion and all its fallacy, mistranslations and lost pages of scripture seems too ridiculous for anyone with access to information and history to believe to be the truth. Unfortunately most people of religious faith were either born into it, or lack the knowledge to make a rational decision to believe or not to believe.
You can convert to any religion or if not you can believe in a divine being like God with out having been raised to. Belief of God is in ones self and is for one to decide. Your relationship with God is between you and God and people can't really effect it.
I have some friends who were raised drug addicts and other "interesting" lifestyles who are now strong Christians. Even kids who grew up in households that believe in God that eventually have to decide for themselves what they believe. Since it is all about a personal decision, upbringing has really very little to do with it.
I was raised to believe in God, and yet I do not believe in God. I never have, even while I was being taught to believe in a Christian version of God. Neither do I believe in any other gods, goddesses, creators, masters, spiritual kings, prophets, etc. So why would it not follow that if you were NOT raised to believe in God, you couldn't still believe in God? I have many friends who are of different faiths who came to their beliefs not through upbringing, but through other personal experiences they have had.
Religion brainwashing has only been used on parents to influence their kids back in 1500, it has being incredibly successful as religion is still around and is still used to give money to the government. You can only be influenced to believing in a religion is by a fatherly or motherly figure.