A piada é provavelmente sobre " Bíblias de edição em letras vermelhas "
Red letter edition bibles are those in which the Dominical words—those spoken by Jesus Christ, commonly only those spoken during His corporeal life on Earth—are printed rubricated, in red ink. This is a modern practice derived from the art and Roman Catholic practice in mediaeval scriptoria of rubricating headings, leading letters of sectional text, and words of text in manuscripts for emphasis, similar to italicization. Red letter editions are not to be confused with the Red-Letter Christian movement, which emphasizes the teachings of Jesus Christ in the Bible, particularly regarding social justice.
Porque a letra é de Jesus, palavras ditas por ele, está em vermelho.
Foi confirmado pelo autor.
(clique para ampliar)
Yep, it was a joke about Bibles where the words of Jesus are in red ink