Nor should there be such a word. For adults, at least, it’s normal to not believe in the existence of Santa Claus. Nor do we have labels to describe people who don’t believe in
Elves
Levitation
Squared-circles
Talking apple trees, or
The claim that Elvis is still alive.
What if those who were not convinced that Santa exists (because there’s no evidence that Santa exists) had to go around labeling themselves as “aclausists.”
In a society where most people professed that they believed in Santa (despite the lack of evidence), the aclausists would be conspicuous by having to refer to themselves as something different every time someone mentioned that “Santa was coming to town.”
This need to repeatedly make this distinction would be a subtle yet powerful signal that the aclausists were different, not fully accepted in the group. Over time, this difference from the statistical norm would take on connotations of “lack of morality.” Aclausists would thus be excluded from political office, teaching positions and other important roles, thus isolating them further. Santa-ism would be a wedge-issue.
Or, perhaps, a fair and rational society could just have no word at all for those who don’t believe in Santa, because people shouldn’t believe in something for which there is no evidence.
"What if those who were not convinced that Santa exists (because there’s no evidence that Santa exists)…"
Huh? Then who drinks the milk and eats the cookies? Isn't the empty glass 'evidence?'
I often wondered who was drinking the milk and eating the cookies on Christmas Eve. Therefore I bought an expensive camera system with infrared triggering. I set it up on Christmas Eve last year and I waited until the morning. Someone actually did sneak in and eat the cookies, but it wasn't Santa (because there isn't a Santa).
The photos revealed the true culprit: It was the Easter Bunny.
Just as diety-worshippers claim that athiests have no structure upon which to base their morality, I guess that santa-worshippers will claim that aclausists have no structure upon which to base their shopping.