In this world, people struggle to tell the truth even to themselves. It makes sense, then, that Emmit tells Gloria that the greatest trick the devil ever pulled was convincing the world he didn’t exist. Obviously, this describes Varga to a tee, but in this case I think Emmit is talking about himself, or the evil within him that prevented him for so long from understanding the truth about his behaviour. When he regains consciousness after he is knocked out by Varga’s henchmen for pointing a gun at him, Emmit notices that a stamp is stuck to his forehead.