"What if you sold 'Harry Potter' books or 'Lord of the Rings' books, but you told people it was real?" Max Rossi /Reuters In February 2012, Trevor McKendrick was out at a dinner with family when he learned that a relative of his was making $8,000 to $10,000 per month selling apps in Apple's App Store.

McKendrick figured if his relative could do it, so could he. He decided to build an app and sell it. His goal was to make $600 per month — enough to cover rent.

He looked at the top-grossing apps in the Apple App Store. He wanted to find an app that was making a lot of money "but sucked."

He found a couple of apps that exactly fit the bill.

"It turned out that there were a few Spanish Bible apps that were terrible," McKendrick told host Alex Blumberg in a recent episode of Blumberg's great podcast, StartUp.

McKendrick hired a Romanian programmer to build an app for him. It sold OK.

Then McKendrick built an audio version of the app. It was a big hit. "That was the moment," he said, "where it was: Oh this is not just a side project. This is a living."

After the audio version was first released, McKendrick started making $5,000 to $6,000 per month.

According to McKendrick's blog, the app made $73,034 in net revenue in its first year. In the second year, that number grew to $100,134.

McKendrick told Blumberg he remembered exactly where he was when he first realized how much money he was making.

He turned to his wife and said, "Holy crap, honey, look at this money."

McKendrick told Blumberg the income "doesn't feel like real money because so little work is involved.

"Dude, I spend maybe an hour a month on this thing."

McKendrick says the money has "changed our lives because I don't have to worry about our income."

Though McKendrick doesn't have to think about money, his booming Bible-selling business does have him worrying about one thing: his morality.

See, McKendrick isn't exactly a believer in his product.

"We don't believe in Christianity," he told Blumberg. "We don't believe in the Bible."

Trevor McKendrick. Trevor McKendrick/Twitter

"I would describe myself as an atheist."

McKendrick was raised a believer — a Mormon, actually. He attended a Mormon college and got married in a Mormon temple. But when his brother left the church he realized he had long harbored doubts himself, and he soon quit, too.

Now he has some guilt about making so much money from selling a religious book in which he doesn't believe.

He explained why to Blumberg.

"What if you sold 'Harry Potter' books or 'Lord of the Rings' books, but you told people it was real? And you told people if they would just learn how to write spells themselves, they could heal their children? And if you sold that as a real thing? I would feel terrible about that. But that's really the situation I am in selling the Bible. I am selling this thing I truly believe is fiction."

Blumberg says he sometimes gets emails from users asking him to pray for them or interpret the Bible for them. "They think I'm a preacher," he says.

"If you're emailing the maker of an app to get help for your son, you're probably not a in a great spot. Trading that for profit weighs on me a little bit."

But does McKendrick feel guilty enough to stop? Blumberg asked him.

McKendrick's answer: "No, I can't. Yes I have a problem with it. But I can't."

The money is too good, and McKendrick really needs it these days. He's using it to fund a new company called BackOffice — which makes a product he actually believes in.

(By the way, if you're into the world of startups and you're not listening to StartUp, you're missing out!)