The obvious answer is that he/she was a child, and simply received the religion from the parents. As far as I can tell, most people are religious because their upbringing was, and of those most simply adopt an interpretation of the religion they were brought up under.

Even in adulthood I'd be absolutely astonished if even 50% of any major religion have read to completion the core text of that religion, indeed I would expect the number to be far lower.