"The Biggest ruse of the devil is making us believe that he doesn't exist," said Baudelaire. Except the devil does not exist. The ruse here only makes sense within a very specific framework; Christian mythology.
There is a meme, a lie, really, within much of the Christian world that atheists are Satanic, or at least deceived in this Baudelairesque fashion. According to this meme, the devil is a liar, thus atheism is a lie. This idea is rooted in Christian scripture where Satan is seen as the father of lies:
43 Why do you not understand what I say? It is because you cannot accept my word. 44 You are from your father the devil, and you choose to do your father's desires. He was a murderer from the beginning and does not stand in the truth, because there is no truth in him. When he lies, he speaks according to his own nature, for he is a liar and the father of lies. 45 But because I tell the truth, you do not believe me. 46 Which of you convicts me of sin? If I tell the truth, why do you not believe me? 47 Whoever is from God hears the words of God. The reason you do not hear them is that you are not from God."
So, if you accept this mythology as true, or at least inspired by a god that tells the truth, then thinking of atheists as living a lie makes sense, right? Well, I suppose, but let's look at it this way. The devil is a character in a set of stories--the mythology which emerges from the biblical collection of books. This character is, by definition, evil and wrong; the narrative of the story is such that he loses, eventually, and in the mean time his power in the universe is based upon deception and lies. Those who do not believe the story of YHWH and his various acts through "history" are seen as, essentially, conspiracy theorists who do not accept the "truth."
But this myth cannot be stretch onto the terrain of reality. Christianity is simply not true, and to try and live it as real can only go so far before the fabric tears and reality pokes through. But for those who are deluded in this strange and inhuman form or LARPing, those of us who are not playing by the rules of that universe will be pegged as a kind of "muggle" who has been projected as wearing a devil mask.
We can't seriously despute the reality of the story, can we? We can't really actually not believe that God exists and that Jesus is our savior, Mohammed is not the seal of prophets, or whatever Mormons believe, right? Because they are convinced of the truth of their worldview, when we enter their periphery we are forced into their role-playing and seen as playing characters which represent us in their narrative.
We are devils, liars, and we have been draped with a cloth of deception to make us fit into their worldview.
But from the atheist point of view, we are a bunch of people who wandered into their D&D, Harry Potter, or Christian universae where everyone within it is LARPing fully in character. So while we know it's all pretend, from the point of view of the players we have to play the role of muggles or devils, otherwise they have to break character.
Except, in the real world, there are people who don't know they are playing a character, they think that the role-playing is real. And that's religion. Atheism is simply being aware that the script(ure) is just pretend, and we cannot believe that so many people take it so seriously. We simply want them to break character, and enjoy reality.
If you are interested in a great book about the history of the concept of the devil, see Gérald Messadié's wonderful book A History of the Devil.