Calvin has struggled with his sin all year. He knows that he doesn’t deserve anything from Santa for all the grief he has given his parents, his teacher, and Susie next door. Santa has every right to ignore him this year and fly on by with his reindeer.
And yet, presents for Calvin appear miraculously under the tree. Note Calvin’s relief at not getting left out. He knows that it could have been otherwise, and he’s thankful almost unto tears for the second chance (He also may look just a little bit triumphant, and this worries me. It suggests that he may take this as license to pelt Susie with snowballs after the toys are unwrapped).
Notice also that the source of the presents, presumably his mom or dad, stands just outside the frame of the cartoon. I have no doubt that Watterson meant it as a metaphor for God (you may interpret Santa Claus, but you’d be wrong) and the presents a metaphor for grace. A free gift, not coming from one’s self, lest anyone might boast (Ephesians 2:8-9). Whatever Calvin does next, the offer of Grace is still on the table.