One of the stranger bits of evangelical Christian subculture here is a movie called Fireproof, which blends marriage counselling with propaganda for Jesus, against a background of huge whooshing flames. It was released in late September, has grossed $32m and is still showing in hundreds of theatres. Made by Sherwood Pictures, a "ministry" of the Sherwood Baptist Church in Albany, Georgia, it was written by associate pastors Alex and Stephen Kendrick. The Love Dare, a book spun off from the movie, hit the New York Times bestseller list in the category paperback advice. So, last week, putting on my breathing apparatus of scepticism, I went to see why anyone would enjoy it.

Because I missed the beginning of Fireproof, I stayed for part of another showing. The second scene is a rant, with a phrase that I knew was repeated throughout. A veteran firefighter reproves a rookie: "You never leave your partner!" The rookie makes excuses, but the captain won't accept them. "Never leave your partner, especially in a fire!"

I cringed, trapped by my own credo: "Never leave a movie, especially if you said you'd review it."

It's the story of Caleb, a fire captain, and Cat, a hospital public relations officer. Their seven-year marriage has degenerated into bitter squabbling about dry cleaning and who has less respect for whom. At work, Cat is flirting with a doctor who plies her with compliments, flowers, and greeting cards. Just as they are planning divorce, Caleb's father, a kindly fellow who has been annoying Caleb with news about Jesus's love, persuades his son to try once more. (This is a man who doesn't hesitate to emphasise his points by leaning on an old wooden cross, which happens to be standing around.) He gives Caleb a handwritten book with a 40-day agenda, titled "The Love Dare", which concludes each entry with a Bible verse. Caleb pessimistically complies.

Kirk Cameron, playing Caleb, makes excessive use of a facial expression in which he appears to be trying to bug out his eyes like a toon. "The Lord saved your marriage?!" "You were married before?!" "Mom did this?!"

Caleb spends a lot of time viewing internet porn, which upsets Cat. She confides this to her mother, who has been left totally aphasic by a stroke, and can only blink supportively. This leads to a gripping scene in which Caleb, while viewing relatively innocent worldly-things porn (photos of a boat he's saving for), is tempted by actual sex porn in the form of a pop-up ad. Will he click to open or click to close? He hesitates, he walks around, he asks himself rhetorical questions, and he seeks guidance in the book his father sent. Reading that pornography, like drugs and gambling, is a marriage-destroying parasite, he bugs his eyes and rushes to the computer.

Expecting him to close the pop-up, viewers instead see him carry the computer outside and beat it to death with a baseball bat. This is enjoyable, although I wonder how he will receive email from the fire department, let alone visit www.fireproofmymarriage.com.

Silly though this is, I suspect that the film's success is because it takes seriously the topic of keeping a marriage going. Not how to meet someone, not how to choose between two people, but how to keep the relationship happy. Since 1952, this difficult and noble subject has sustained the world's longest running women's magazine feature, "Can This Marriage Be Saved?", in the Ladies' Home Journal.

This always begins with a wife telling how she fell in love with and married the husband – and how dreadful he has become. (Clearly the only hope is for him to change completely.) Then the husband tells how he fell in love with and married the wife – and how dreadful she has become. (Clearly this couple is doomed.) Then a therapist describes how he or she helped the couple get past their troubles, and how they are now happier than ever. (Wow! Clearly the world is more hopeful than we realised!)

Fireproof bungles by omitting the part in which the partners tell how they fell in love. We meet Caleb and Cat as surly whiners. We have no idea why they married. Who cares if they divorce?

In the big fire scene, Caleb (leaving behind both his partner and his radio) rushes into a burning house to save a little girl. While he is finding the unconscious kid, seeing his retreat blocked by flames, tugging on a barred window, and chopping a hole in the floor so he can get down into a crawl space with the child as the roof collapses, his buddies are standing out front shouting for him. They never walk around the house to see him tugging at the barred window, or even around the side of the house where he forces his way out of the crawl space.

If they had, there would have been less drama. But this leaves it open for me to point out that if you only look at things from one angle, there's a lot you don't see. There can be more than way out of a burning house. There's more than one way to fix an injured marriage. But you won't be surprised to learn that Caleb finds Christ, spends his boat savings on medical supplies for Cat's mother, and wins Cat back. They remarry in a "covenant marriage".

I'd say Fireproof's success is due to shrewd marketing, a widespread wish to watch people mess with fire, and the fact that marriage is a powerful subject. But the filmmakers might disagree, having told The New York Times, "The only way that this could happen is if after we prayed, God really answered those prayers."