Why are you seeking to “convince” a 12-year-old of God’s existence, as if you are required to make a case and argue with him over the matter? If this child is a student, assign him age-appropriate homework and require him to make a case for God’s existence to the class. If this child is your own, lift an eyebrow and say to him bluntly, “You are still in religious and moral formation, young man. If you have reached the conclusion that God does not exist, then it is apparent that your father and I must redouble our efforts in your religious education and require you to start attending daily Mass.”
The point is that teachers and parents really must stop becoming intimidated by these kind of announcements from children under their authority. Children like to shock adults, but they also want adults to reinforce the boundaries of their world. If, instead, adults act as if they cannot handle childish rebellion, the child will become insecure and fearful. What may have started as a testing of the waters will become a full-blown religious/moral crisis in no time.
