Since we are using the english language, and since it becomes a metaphor, you can change up the words simply and have the phrase be of actual meaning to atheists (people who dont believe in god).
The Bible, (or any religious book for that matter), have been motivators throughout the millennia, making people establish religions and forcing people to turn into their religion and bash any other religion other then their own. Lets switch Bible with Cake
The Bible is a lie.
Now this can be a simple phrase to utter, but cannot be an actual truth until we find some actual way to prove that the bible doesn't exist, (other then all the other obvious ones that got religious people completely ignorant). But can there be such a metaphor or such a comparison created for two completely different things? Possibly.
But if you are religious and are shaking your head in disapproval right now, but are a really big Portal fan, (This is to the what, 6 people out in the world?), think of it this way, if we use the same phrase.
Atheism is a lie.
Are you sure? Atheism was not as much motivating as any of the other religions were, since Atheists were not begging to burn witches, or create churches in their religious honor, (at least around the 1800s, I haven't seen an atheist church but tell me if they actually exist), so due to the phrase, you can think that Atheism has a less chance of being Cake aka a lie, then Christianity does. But also once Atheism is proven to be fictitious, then Atheism becomes cake.
Therefore the percentage of Atheism being a lie (just by using this phrase and no other form of hypothesis) is lower then the chance of Christianity or any other religion being a lie, since those religions already are filling the cake up and putting it in the oven. When it is proven ficticious, then there is the icing on the cake, (no pun intended). Either way, there is your weird metaphor that I just thought up. I have no idea if this is logical or not, but its a thought to toil around in the back of your brain. Its been 5 hours and I still don't know.