Panel 1: It's Christmas Night at the Pattermanse and we have a wide panel of the family all bug-eyed and feeling hors de combat after stuffing themselves all day long. As they try to digest their too damned big a turkey again, John says that the song is right because every year, they glory in excesses.
Panel 2: We zoom in on Liz as she asks Daddy "Isn't that glory in excelsis?"
Summary: It isn't quite that, of course. What we're looking at is John's passive-aggressive bitching about having to spend too much money on things he wouldn't buy if it weren't Christmas. After all, the dope's anti-materialism sermonising seems to me to stem from his disdain for spending his money on other people to get them things he doesn't like.