Of course, while Once Upon a Deadpool might be a PG-13 version of the movie, the film is still a Deadpool movie.
There were even rumors before the release of Deadpool 2 that the film was going to be subtitled The Second Coming, so it’s possible the studio has had this poster idea kicking around for a while, and just decided that it made sense to use it for the Christmas time release of Once Upon a Deadpool.
It’s not like everybody involved didn’t know what they were doing when they chose that particular religious image as the basis for the poster.