To what extent is it true that everyone is born upon fitrah?

I often recall my memory from the past when I was first recognised God. When I was a child and misbehaving and my mother said "Remember God is watching you" and reply "Who's he(God)"?. That memory casts me doubts regarding whether it's true that everyone is born with instinct to believe in God. I often think that is it possible that it's not true for everyone and often think that could it be the case that some believe in God not because it was in their instinct but that is what they were told since their childhood?