How can we make adults believe in Santa?

[Smaug]

I've been wondering why Santa belief disappears but religious belief doesn't. What would we need to do to turn Santa belief into a religion that could compete with Christianity, Islam, etc.?

Here are a few changes that might help:
(1) The real-life gifts on Christmas need to change, because that is too easy to falsify. Either the gifts need to be invisible, or the gifts need to arrive at unpredictable times as random good fortunes. If you have not been naughty, but you still are not receiving gifts, then you simply don't appreciate the gifts that you are getting, and that is very naughty. That explains why you are not getting gifts.
(2) The cookies and milk need to change. Every Christmas Eve, the Santa believers consume a cookie and drink some milk with the assurance that Santa will share this snack with them in spirit.
(3) Santa needs a hell that depends on belief. If you stop believing in Santa, then you will remain on the naughty list until you repent of your unbelief. If you never repent, then you will burn forever in Santa's cookie oven.

You've forgot the Enemy. There has to be the Enemy to pile all the mishappenings onto. Say, Rudolph the reindeer, who had coveted Santa's divine ability to bring presents and betrayed him by misleading the sleigh on one Christmass night. The sleigh fell down from the skies somewhere near the South Pole and only Rudolph and Santa survived. Rudolph rammed Santa with his mighty horns and shot laser beams from his nose but Santa in his divine wrath pulled out a magic candy cane shaped steel rod and drove the traitor into the deepest caves leading into the very center of Earth (widely known to be flat AND hollow) where he still resides and mines coal alongside with Hitler and his jewish-reptillian illuminati nazis from Nibiru.