I would tread carefully lest I upset either of the child's parents. But in your position I would like to tell him that over the millenia religion has proved to be an efficient control mechanism for deluded and/or unscrupulous pacifists and warmongers in their campaigns to subjugate the terminally gullible.
But what if Abacus's religion isn't the "one true" one?
Isn't he/she correct then, by your lights, not to have faith in it?
(BTW, Tom, which one is the "one true religion" - it's hard to tell from your description, since that claim is the one thing that most of them have in common)
Tell him to lead a good life and not to worry about different religions as no man is or ever was truly capable of understanding all God has to say.
That way you can explain the differences between religion as differences in interpretation between differing cultures and groups.