Christopher's teacher Mrs. Forbes tells him that his mother went to heaven after she died, but he doesn't think heaven exists.
He once asked a priest where heaven is and was told, "It's not in our universe. It's another kind of place altogether" (61.3).
Since it's impossible for anything to be outside of the universe, Christopher concludes that heaven cannot exist, and that "people believe in heaven because they don't like the idea of dying" (61.6). Well, then.
Christopher thinks that when people die, they just die, and they're either buried and decompose or are burned and turned into ash.
His mom was cremated, so sometimes he looks into the sky and imagines that the molecules of her body are floating over the earth and coming down as rain or snow.