For a moment I thought a friend of mine had become a Mormon. There in my news feed on Facebook, between posts from people I barely know and photos of babies (I'm 30), was a status update that read: "No matter how bad your past has been, you can change and move on."

How nice! But the post wasn't by or even endorsed by anyone I know: it was an advert for the Mormon Church placed in my feed (where I don't remember ever seeing advertising) as part of an online mission of such scale it would be the envy of its most prominent believer's presidential campaign.

After 200 years of pounding pavements all over the world, the Church of Jesus Christ of Latter-Day Saints, which counts Republican candidate Mitt Romney among its 14 million followers, has put its faith in digital door-knocking. Its Facebook page has Jesus as its profile picture and "likes" by almost 650,000 people. A YouTube channel has scored 40 million video views, while the church's UK arm calculates that its own Facebook page "reaches" 900,000 people.

Jehovah's Witnesses, by contrast, another big missionary religion, boast seven million members but their site does not even have an associated Facebook page.

The Mormon reach can only now broaden thanks to the new ad campaign, which started last month. Facebook says it is part of an "ad unit test", or trial that places marketing in the news feeds of people who have expressed no prior interest in a brand, be it a restaurant or a religion. Facebook would not say which or how many advertisers are taking part, nor how much ads cost to place.

The spot in my feed includes a link to Mormon.org, the church's impressively slick website, and a video of a man called Stan Checketts, a rollercoaster designer from Utah who throws axes and talks about finding faith. Stan is part of the Church's "I'm a Mormon" campaign, which it launched in the US in 2010 with poster ads. They included one outside the New York theatre staging The Book of Mormon, a satirical musical from the people who brought you South Park. It comes to London next March.

The church has said it seeks to challenge stereotypes but wants primarily to spread the good word. The adverts are targeted at "everyone, especially those who have questions like 'Who am I?'," a spokesman in the UK says. "We'd like to explore the purpose of life with them and help them discover God's plan through the teachings of Jesus Christ."

He did not reveal how many people had done so thanks to the ads, nor what the digital strategy has cost. The church, which is as much a corporation as a religion, is worth an estimated $40bn (£25m). It has its own digital-media firm, and, in the UK, 15,000 acres, including two temples, in London and Preston (there are 200,000 Mormons in Britain).

The Mormons possibly arrived at the wrong virtual doorstep with its ad in my news feed but, intrigued, I decided to open up by taking part in a missionary chat. The church says it has conducted a million such live conversations in the past year.

I was soon joined by Seth and Tim. See the start of our conversation, above. Later on, Seth offered to give my phone number and address to a missionary in my area. I politely declined, unconvinced by Mormonism, but rather in awe of its digital powers.