In many works, the religion of the characters is simply never mentioned. They might have the same religion as a plurality of the audience. They might have no religion at all. It just doesn't come up.
Used as a way to appeal to a broader audience.
- Word Of God from Stan Lee is that he consciously never put religion into his work. Is Peter Parker Jewish? Christian? Atheist? He could be whatever you are, dear reader.
- Played with in The Good Wife. The lawyers at Alicia's firm seem to be playing the trope straight, but in some cases (such as Alicia herself) the script goes so far as to imply they're actually irreligious. Peter and Grace both get Born Again (separately), whereas Jackie's mainline Christianity and Eli's Judaism may be more cultural than spiritual.