The Spirit of Christmas is the name of two different animated short films made by Trey Parker and Matt Stone, who would later become famous for creating the animated series South Park. To differentiate the two, they are often referred to as Jesus vs. Frosty and Jesus vs. Santa. Both these clips are NSFW
Jesus Vs. FrostyIn 1992, Trey Parker and Matt Stone, then students at the University of Colorado, made Jesus vs. Frosty, under the "Avenging Conscience Films" moniker. Parker and Stone animated the film using only construction paper, glue, and a very old 8 mm film camera, and premiered the film at the December 1992 student film screening. The movie features four kids who are very similiar of the four main characters of South Park, including a character resembling Cartman but called 'Kenny', as well as a hooded boy resembling Kenny and two other nameless boys resembling Stan and Kyle. The film also includes some classical South Park ingredients, like an absurd story-line, strong language, graphic violence and a 'moral' at the end.
Santa vs JesusIn 1995, Fox executive Brian Graden paid Stone and Parker $2000 to make another animated short as a video Christmas card he could send to friends. In turn, the duo created Jesus vs. Santa. This version of The Spirit of Christmas featured an animation style very similiar to South Park, as well as more developed versions of Stan, Kyle, Cartman and Kenny (and a cameo appearance of a girl looking exactly like Wendy Testaburger, sitting on Santa's lap). It largely established the characters as they would be used in South Park. The movie also contains elements which would re-occur in the series, like Kyle being Jewish, and rats eating Kenny's corpse. The film reportedly had a budget of $750, with Parker and Stone keeping the rest of their commission. The making of the short was parodied in the South Park episode "A Very Crappy Christmas".
Descriptions of the episodes have been harvested from Wikipedia.
See also South Park's rise to fame