The Invention of Lying/WMG

God is responsible for Mark's ability to lie.
Before Mark starts lying, the entire world has no concept of religion or fiction whatsoever. Then Mark suddenly comes up with something suspiciously close to an Abrahamic religion. That seems different from lying about your bank balance.

Mark is a prophet. His "lies" about God and the afterlife are true and a genuine religious revelation. His ability to lie is a power granted him by God to help spread his message, while at the same time testing his character.

Mark was by no means the first liar in history.
There were many liars among the ranks of the great historical figures, and they were great because they could lie. Because nobody was ever able to call them out on lies, they wrote history with those lies. It may be possible for the world to rely on a select few capable liars to exist. Mark is just the newest amongst the ranks of liars. ""Hitler, are you going to cause another World War and kill millions of Jews, disabled people, and other "undesireables"?" "Uh. Nope.""
 * This would explain why history hasn't radically changed from ours...

"1. Just because there is no lying doesn't mean there isn't killing (after all, the masses still understand what he's on about when he's saying don't kill people). 2. Whilst Hitler did rise through the ranks before becoming a dictator, take note that being wrong isn't the same as lying. Bigots like Hitler just had to be very persuasive, because if you believe a stereotype enough, it can become the truth to you."
 * Granted, this theory is feasible, but how does the Hitler example work?

The world of The Invention of Lying is the homeworld of the Thermians from Galaxy Quest.
The Thermian culture developed without the concept of the lie. Sure, the Thermians were blobby squid-things, but would anyone have been able to follow the movie if everyone in The Invention of Lying was a blobby squid-thing?
 * Even back then, they had already invented holographic image inducers. Those darn documentary transmissions were still an underground phenomenon, but cosplaying as humans had gone mainstream.

Mark was completely and utterly insane.
Suddenly saying things that did not reflect reality? A sure sign of madness, second only to talking to yourself while cooking your customers into pies. Unfortunately, this talking-about-not-realness affected others, convincing them that it was not only made sense but also was a good idea.

There must be other worlds or timelines where someone fell under this madness far earlier, perhaps at the dawn of language, perhaps inspired by the edible butterflies that resemble poisonous butterflies and get eaten less often than the ones that don't.

Mark is a geass user.
The film shows what would have happened if Emperor Charles zi Britannia had succeeded in his plan to make a world without lying.

All of the people in The Invention of Lying are... those things from Equal Rites
That race that couldn't lie... But then some of them learned to lie... you know? And then they were never mentioned in any later book? Yeah, them.

The Invention of Lying World is an experimental Alternate Reality where a god has limited humanity's freewill in regards to lying.
The ability to lie is a freewill concept where the person choose to either tell the truth totally, partly or not at all. A god-like being for whatever reason decided to remove the ability to lie while allowing them free choice in every other respect and this world is the result.

The world is the far future in a totalitarian dictatorship that stomps out all imagination
Here's how I see it. A long time ago, a totalitarian regime seized control of the entire earth, declaring that deception, religion and imagination is the source of all the world's ills, so they regulated it heavy from a young age, using drugs and conditioning to control it. Maybe even secret executions, too.

All fiction has been destroyed and only things like Lecture Films are left, which started as a propaganda machine to produce state-approved "history". It's been several generations by now, to the point where absolutely no one alive remembers how life used to be, including the people running everything.

However the side effect of this is that there is no creativity or invention, leading to absolute cultural and technological stagnation, there are no new ideas for anything at all, so even though it's been decades, everything still appears to be late 20th century.

Mark was formerly on the "necessary" medication that everyone takes to suppress imagination, but began forgetting to take it when he became too depressed over his life.
 * Or the regime thing was a long time ago and lying became a recessive gene, everybody less or more accepts everything now, and Mark's the first to learn to lie again.

This world is a continuation of Fahrenheit451, with Mark the first person in living memory to be able to lie.
This is a variant of the above idea, with another possible hint to origin. Think about it, all works of fiction are gone, eliminated because their stories were made up and/or contradicted one another. Over the course of the years people would give increasing credit only to what they could immediately see, hear, sense, all in the name of Reality. Books might be kept around as records of facts, but for no other reason; hence only reference books were spared in the Firemen's destruction of all libraries. In time, people might regard saying "a thing that is not", ie, lying, as a serious mental disorder and start inoculating children against it, as you would against smallpox.

This would ultimately apply to even using your imagination, because it's talking about people and things that are not explicitly existent in the here and now. Making up stories, even realistic ones, would become the verbal equivalent of masturbation, and all such tendencies would be firmly culled. Honesty would be considered paramount in all things, even if it's put rudely; somehow the blunt nature of many statements makes them more "true" and therefore more desirable. Being tactful for the sake of someone's feelings would increasingly be seen as a weakness: "Why don't you just say what you really mean?" Within a few generations no one would have known the world to be any other way, and all evidence of the past would be destroyed, first from shame, and then fear. The "truth-telling" virus would be passed down to children and grandchildren.

This would explain why churches and the like would still exist, but have all relics such as crosses removed; people still used them as a place to reflect on things, but have no recollection why. It might also enhance the emphasis of genetics in choosing a mate; in a world where everything on the surface is the only perceived reality, people gravitate toward what they know isn't going to change much, such as DNA. Mark was maybe the first person in living memory whose body began to reject the ingrained "truth vaccine".

Once Mark starts reviving the art of telling stories, people naturally flock to him without remembering why. It would be moreso when he speaks of the Man In The Sky, again, they find it appeals to long-suppressed desires of hope and revering something higher than mere human nature. If Bradbury's Book People were still around, no doubt they'd have long been in hiding, maybe looking for a resurgence of so-called "lying".
 * This might be the far-off Real Life result of doping up every kid with Ritalin, anti-psychotics and Comatonin... you know, for... daydreaming? ...thinking outside the box? ...acting like humans? I'd definitely put this under Poison Oak Epileptic Trees.

Mark has superpowers
Specifically, not only the ability to tell a lie, but also the ability to convince people he is right, despite all the evidence present. This would explain things.