@Outlaw - 'proof' doesn't sit well with me here, but I agree with you that belief in God can sometimes keep teenagers 'on the straight and narrow'.
Cofty and JonathanDrake: extreme scenario - a hopeless alcoholic with a huge self-destruct button starts to have some kind of personal faith in God, and his over-endulgence in alcohol starts to decrease. Would you tell him to his face there is no god?