The Anti-Papists were a Protestant terrorist group organized in Quebec in the 1850s in response to an upsurge in attacks on Anglophones by the Patriotes. The Anti-Papists killed Francophone Quebecois, burned Catholic churches, and, as Sobel notes, were as brutal in their way as the Patriotes. Between 1855 and 1870, the de facto civil war between the Anti-Papists and the Patriotes led to over half a million Quebecois, both Anglophone and Francophone, fleeing the confederation. Presumably the Anti-Papists either gave up or were suppressed after Quebec gained autonomy in the 1889 Quebec Plebiscite.
The Anti-Papists do not have an entry in Sobel's index. They are mentioned on p. 141 of For Want of a Nail ....