Belief is not a static thing. Sociological research shows that, at least in the United States, church affiliation (taken as an external indicator of faith) ebbs and flows throughout a person's life. It has become statistically normal for people brought up in a religious household to experience a crisis of faith in adolescence and/or young adulthood and return to the family faith once married and with children in their late 20s or early 30s.
Faith typically springs from one of three sources: revelation (an angel appears to you, which is not very common); reason (your thinking leads you to conclude that the possibility of God, albeit not irrefutable proof, is reasonable -- again, not hugely common); witness (a friend, parent, teacher, etc. tells you about their faith, this is probably the most common). My experience of religious people, which included being in a sociological team surveying on this topic, suggests that most believing people first follow family custom, then face the challenges of reason and sometimes have a revelatory experience (most often quite short of an actual angel).
Thus, believers might legitimately demure when confronted with someone who would pose a question derived from yours, such as: why don't you realize you're just following what you were told as a child? That would be because, in actual fact, their faith has undergone ups and downs and they have drawn on other resources, such as reason and spirituality, to decide to adopt their faith as a matter of conviction.
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