Faith Promoting Lie (FPL): a religious story posing as fact, but with little or no bearing on an actual event; written with the intent to teach a doctrinal principle, often drawing a strong emotional response.
Before I get into the detail of FPLs and how I came to realize their potential (for good and not-so-good), a little background in how I came to know of their existence...
As missionaries for the Church of Jesus Christ of Latter-day Saints, we shared the gospel by teaching the doctrine of Christ out of the scriptures and from the teachings of modern, living prophets. The Holy Ghost would testify to our investigators of the atonement of our Savior and of the truthfulness of what we taught about His gospel. We would also share our own experiences living the gospel and the effect it had on our lives and testimonies.
In my particular mission, many missionaries (including myself) would collect and share posters and stories that related to the gospel. I ended my mission with two 3" binders full of sheet protectors with stories, articles, illustrations and teaching aids. Most of these were for my own enjoyment. They either came from tear-jerker emails that my family and friends would print off and mail to me or other missionaries, who probably obtained them the same way.
Some of these stories were very touching stories. One of the most well-known of which is entitled Seminary Donuts. I cried when I first read the story. Since then I have heard it read by several different people to mostly tearful audiences. It is one of those email forwards that members of the Church love to forward to all their friends -- not for seven years of good luck (like those really annoying forwards), but because they believe it's the most touching and good story they've heard in a long time.
Toward the end of my mission, after I had collected probably a whole binder's worth of these stories, I realized that these stories are lies. Please don't misunderstand: the gospel doctrine they teach is often (at least mostly) correct, but their claim to be a true story is simply false.
This claim of being a true story comes in a variety of forms. Sometimes the story itself gives the state (usually not more specific than that), names, and season that the event supposedly took place in. Other times it just starts the story without disclaimer that it is made up, and when the story is read over the pulpit (even by a high council speaker) people mistake it as a true story when no one (even the speaker) doesn't know the source of the story.
Inevitably, and largely due to the anonymous nature of email forwards, these FPLs have no traceable author. The name, if given, is just a first name, or even a full name that leaves no way to look the person up (since even a full name doesn't help look up a person if you have no idea where they live).
"But wait!" I hear you cry. "[your favorite FPL here] is true! I cried when I read/heard it. Surely no one would make up a story like that. It happened! The story even says where it happened..." Surely we cannot be as gullible as to believe everything we read or hear. Crying in reaction to a story does not mean the Spirit was testifying of the story's truthfulness. The story itself can be emotional enough to bring tears. Perhaps (maybe!) the Spirit itself was even there to testify of the truthfulness of the principles taught in the story, but not the story itself.
FPLs, which I now group with "Mormon folklore", I see as a bane to our religion. Mormons seek after and hold to truth. FPLs pose as truth and can mislead us if we do not discern them. "What damage does it do to believe in an FPL if it teaches good principles?" Teaching good principles is good. Using a story to help get a point across is fine. Even Christ used parables (stories that never occurred) to teach the gospel. But when the teller claims that the story actually happened when in fact it did not in order to help make the story more powerful -- that's further than Christ went and I believe is damaging to the Spirit and the delicate faith of the Saints. Also, as long as the story is made up there is no guarantee of the purity of the principles taught. Scriptures make a much more sure source of truth.
Let us be watchful. I am not suggesting that we become constant skeptics in our sacrament meetings or of missionaries. But I am suggesting that people can innocently convey falsehoods that have nevertheless touched their lives and that they want to share with others. If you hear a story that touches you that you want to remember and perhaps even share with someone else, please take a moment and ask the source for their source of the story. If it is a personal story of their own, ask for permission to share it with others if you would like to. If they got it from somewhere else, see how much documented evidence of the original source you can track down. If it's an FPL, you won't get far. And you're better off teaching verifiable truth from the official Church materials.