Regarding the point about upbringing not having a lot to do with a person's belief system, I think that is false, here’s why. First, your upbringing moulds and shapes you to a certain extent in how your parents view life’s problems and how they deal with them. Parents are rarely objective in their passing down of things like rules, experience, truth, etc. What they have learned from their life and from their parents is inevitably dumped on their children. Religion is considered by many (including me) to be the most important of question lists as it attempts to answer why we are here, if god is real, what happens when we die, etc. Therefore as the parents hold a certain belief it can be a sure bet that their child will also hold the same belief (although it an extremely passive and innocent way).
This brings me to my second point. A secure and sure-fire method of establishing someone’s religion is to find out where on the planet they were born. You see, belief has more to do with geography than any so-called truth that the doctrines may hold. Muslims largely come from Saharan Africa and the Middle-east, Hindus from the Indian sub-continent, Shinto’s from Japan, Christians from Western European countries. My bet is that if you were bought up in Syria as a child and your parents were indigenous, then you would likely be a Muslim. And you would likely say that Islam is the true way to heaven and the Christians are wrong. So upbringing has an incredible influence on a person’s religion, more than most people realise. In fact it could be stated adequately that one’s religion is an accident of birth.