Seven isn't about the Seven Wonders of the World, or the seven dwarfs, or even a seven-layer guacamole dip. It's about a serial killer acting according to the seven deadly sins—gluttony, greed, sloth, pride, lust, envy, and wrath.
So, it's no shocker that the biggest theme in the movie is sin.
Questions About Sin
Are the seven deadly sins actually deadly if you took the crazy serial killer out of the equation?
Do the seven deadly sins still apply today? Do new sins come with a new century? What new sins might replace these sins that are hundreds of years old?
What are the seven cardinal virtues? Are any of them present in this movie?
Chew on This
Doe believes that "envy" is the worst sin, which is why he has himself killed.
The sins were used to scare people into behaving, and Doe is bringing back that idea of causing terror in the general public.