What is the main twist in the plot of The Man Who Was Thursday?

There are numerous head-spinning twists and turns in The Man Who Was Thursday. But I would argue that the biggest one of all—plot spoiler alert!—concerns the identity of Sunday, the mysterious head of the clandestine anarchist group infiltrated by Gabriel Syme. No one is ever quite who he seems in this dizzying tale of bluff and double-bluff. But then again, the story involves an anarchist group bound together by rules and a rigidly hierarchical organization, so it's not really all that strange for there to be such a profusion—not to say, confusion—of identities.
It turns out that all the members of the anarchist group's governing council are, like Syme, undercover detectives. What's more, Sunday himself has been working undercover to thwart the anarchists' fiendish plans. Far from being a wicked, Machiavellian figure, Sunday is actually a rather decent old chap—from Scotland Yard, no less—who's presented by Chesterton as being very much on the side of the angels.