Mel Gibson's controversial The Passion of the Christ is doing well at the box office. It cost about $30 million to make the film, and it quickly generated more than that in box-office sales once it premiered, becoming the highest-grossing movie with a Christian theme. Gibson stands to make several hundred million dollars when all is said and done. To those who didn't realize it before, religion-oriented products are big business.

According to a recent Forbesreport, firms that do a lot of Christian business include Wal-Mart (NYSE:WMT), Sears (NYSE:S), Target (NYSE:TGT), Best Buy (NYSE:BBY), and Trans World Entertainment (NASDAQ:TWMC). Indeed, it said that Wal-Mart now offers 550 different Christian music titles and more than 1,200 Christian book titles. The Christian book industry is estimated to be worth $4 billion. The overall video game industry is roughly a $10 billion one, and as Christian video games grow, they're poised to take a chunk of that, though the chunk is currently only $100 million or so. (One Christian video game is N'Lightning's Catechumen.)

Religion is a factor in more than entertainment. A Christianity Online article points to the dieting industry, which weighs in at $30 billion to $50 billion and sports a Christian segment of at least 5% (amounting to $1.5 billion or more).

If you're interested in investing in companies offering religious products or perspectives, a little digging will turn up firms such as Thomas Nelson (NYSE:TNM), a major Bible publisher, and ServiceMaster (NYSE:SVM), parent of Merry Maids, Terminix, TruGreen ChemLawn, and Rescue Rooter.

Meanwhile, if you're wondering what Fools have to say about Gibson's film, our Religion & Culture or Great Movies discussion boards can give great insight (try them free for 30 days). Just to give you a taste, in a thoughtful post, Fool Community member dsracic thought the movie "was quite possibly the biggest non-event I've ever experienced." It got more than 150 responses.

Longtime Fool contributor Selena Maranjian does not own shares of any companies mentioned in this article. But she did go to church this week.