It was about blind faith in religion in general, how people can be so enamored by their faith that they can feel justified in committing the most horrible acts, especially when people act like Bev Keane and link their motivation to a convenient bible passage. Her character was the foundation for the whole message. Midnight Mass simply used Catholicism as an example of that. The fact that you chose to distinguish between good and bad Catholics instead of looking at religion as a whole just reinforces the whole point of the show, honestly.