Gervais plays Mark Bellison, the first man to "say something that wasn't". This use of a virtual margin radically transforms his life, making all sorts of things possible that weren't possible before. He gets the girl, persuades his dying mother that dying isn't so bad after all because there's a heaven, and as a result becomes the prophet of a new religion which tells people what they want to hear. Gervais' point is both anti-religious (since religion is clearly the biggest lie of all) yet also pro-lying (since lies make life bearable) -- which makes it pro-religious again, in a sense. Religion is untrue, but legitimately comforting.
From the glosses I've read in reviews so far, it sounds to me as if The Invention of Lying melds the themes of The Misanthrope and Tartuffe, both by the great 17th century French dramatist Moliere. The world of the early part of the film, a world without lying, is very much the world of Alceste, Moliere's "misanthrope", who's unable to lie to spare anyone's feelings, unable to flatter to capture the heart of a lover, and unable to compromise to advance his own career. In the end he decides to retreat from society altogether. The second part of Gervais' film is more Tartuffe-like, and concerns the hypocrisies of religion.
Since this is a comedy aimed at -- and set in -- America, The Guardian thinks its presentation of religion as a big lie is "rather radical... It's one thing for Gervais to air his atheism on the standup circuit. It's quite another to do so in the guise of a glossy, user-friendly sitcom pitched squarely at the huddled masses in the American multiplex." It's rather a depressing thought that things that were vaguely subversive in 17th France are still considered subversive in 21st century America, but worth remembering that the Alceste character in The Misanthrope may be Moliere's auto-critique (Alceste's honesty gets him tangled up in various lawsuits, which Moliere also was at the time), but also that he may be a parody of the protestant and puritan attitudes which at the time were just beginning to chafe against Catholicism.
You can see how Protestant and Catholic attitudes to lying might be rather different. Just seventeen years before The Misanthrope was produced, for example, a puritan dictator called Cromwell had seized control of England and closed all the theatres, because plays were dangerous "lies". In the Protestant, puritan imagination we have a direct personal relationship with God, and make accounts of our conscience to Him. In the Catholic imagination, there's more room for slack and embellishment. We can do bad things -- everybody does -- then confess them through the mediating machinery of the church, and be absolved. Post-catholic cultures, as a result, go in more for compromise, for private confession, for compassion. They're more forgiving of "hypocrisy", for keeping a split between public and private life. They're less keen on direct confrontation, litigation, condemnation, and moral witch-hunting. There's a reason Arthur Miller did not set The Crucible in France.
As Ricky Gervais' film shows, lying gives you a particular edge in a world where everyone tells the truth. In a world where everybody lies, clearly the playing field is effectively leveled and the advantage of lying much reduced. Even in that world, though, there might be good reasons to lie. First, lying takes a bit more effort, a bit more ingenuity and a bit more work than telling the truth. And (dis)honest hard work is of course a prime value in itself. Secondly, as Bellison shows when he softens his sick mother's death, lying can be compassionate. We tell people -- and ourselves -- lies every day to soften hard blows and generally be kinder, something puritan truth-telling prigs might do well to consider more important than simply stating harsh, bare facts. Thirdly, as Gervais also points out, lying can be creative, productive, dynamic, transformative. Lying isn't just "saying something that isn't", it's a way to propose a new and different arrangement of things, one which occurred in your imagination rather than in the real world. Once you've proposed it as a lie, you can work on making your lie true.
Next time you hear someone spitting contemptuously the word "Liar!", try this thought experiment. Imagine that same person, in a parallel world, cooing the word in a truly impressed, admiring voice. Now you're in a parallel world where liars are heroes. Look around, try things out. What's it like there? Is it better or worse than the world we know? Or is it exactly the same? Do we perhaps already live there, and just not admit it?