Ofcourse that's fully possible. My parents was Christian. They raised me a Christian.
It wasn't until my late teens, when I got a job that I was exposed to a critical thinking crowd. They were very much willing to speak their minds, instead of showing religion some special-cased "respect".
It made me think. It made me reconsider. I'm now an atheist.
But oh boy was the period in between then and now a interesting one. Somehow just asking yourself "what if there is no God?" felt so incredibly wrong and the mind many, many times tried to stop itself thinking that very thought. Confusion is certainly not a sufficient word to describe what went on then.
Anyway today I'm extremely thankful that people didn't show me nor my religion "respect". They treated me with reason, rationality and critical thinking even though they knew it might offend me. Now that is showing someone real respect!
Exercise — of both body and brain — is work. Not having done it does not mean one is unable to.
Many live entire lives without a taut muscle. Yet we know they could have lived differently.
If we look at someone who is born into a religion, lives their life holding those religious beliefs, and dies as a member of that same religion - is it valid to assume that they could have separated themselves from their beliefs? Just because a subset of the population undergo religious conversion or deconversion later in life (for whatever reason), does not imply that everyone has that capacity. It may well be that for those people, their religion is effectively immutable.
(Also, racial characteristics are at least partially alterable, using plastic surgery to modify physical ethnic traits, e.g. see http://nymag.com/thecut/2014/07/ethnic-plastic-surgery.html)