You are right, people don't generally choose their religion, it's choosen for them by their parents. It's not until later in life that you acutally get a choice. By then, the religion you grew up with is so engrained into you that change isn't likely.
This is one of my questions to those that believe that "their" religion is the "true" one. When the only reason that they believe the way they do has to do with what they were taught when they were younger.
If you seperated twins at birth and put one in a devoutly Christian home and one in the home of a devout Muslim, they would both grow up believing that their religion was the true one. But they can't both be right, so who is? Would you be able to get them together and agree on who is right and who is wrong? Like I said, it's all about how and where you were raised.