If you find yourself short of cash, perhaps consider taking a leaf out of Trevor McKendrick's (good) book – he built a Spanish Bible app and now makes over $100,000 a year. The most surprising part? He's not a Christian. In fact, he's a self-confessed atheist.

"We don't believe in Christianity. We don't believe in the Bible," he told StartUp podcast host Alex Blumberg.

"I would describe myself as an atheist."

McKendrick was raised as a Mormon, and even got married in a temple, but left the faith after realising he had doubts. Now, he says he feels guilty about profiting from a book he doesn't believe in.

"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," he told Blumberg.

"But that's really the situation I am in selling the Bible. I am selling this thing I truly believe is fiction, but other people are trying to use [it] and mold their lives to fix large and small problems."

However, the money is too much of a pull for McKendrick to consider putting a stop to his app, La Biblia Reina Valera, developed by his company Salem Software LLC. He is using it to fund other parts of his business.

He says that the idea first came after he discovered that a relative was making up to $10,000 a month selling apps. He decided to build one himself, and discovered a gap in the market for a Spanish translation of the Bible.

"It turns out that most of the Spanish Bible apps out there are really bad. (I should point out there are a few exceptions to this from competitors that I admire)" he writes on his blog.

"My hypothesis was three fold: (1) Whoever was making Spanish Bibles right now was making decent money, (2) I could make a better Spanish Bible app relatively cheaply, and (3) the competition wasn't too heavy so I'd still be able to be found."

He initially created a simple app, but later expanded to include an audio version. Though he spent just $500 on the first design, he was soon making up to $6,000 a month.

McKenrick said that users sometimes emails him with prayer requests, or even to interpret the Bible for them.

"They think I'm a preacher," he explained. "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."

Listen to the full interview below: