The particular Protestant tradition in which Samuel Clemens was raised had a strong anti-Catholic bias, though his own relationship with Catholicism was complex. Innocents Abroad displays a good deal of contempt for the "superstitions" of Catholic cultures in the Azores and Italy, and explicitly sets the Catholic Church apart from "the only true religion, which is ours" -- i.e. Protestantism, which was the predominant form of American Christianity throughout the 19th century. On the other hand as owner of Webster & Co, MT enthusiastically published The Life of Pope Leo XIII in 1887. His enthusiasm was admittedly more commercial than ecclesiastical -- he apparently felt every Catholic household would feel obliged to buy a copy, and was very disappointed with the book's weak sales.
Given MT's desire to be popular, religion was always a delicate subject: he instructed Fred Hall, his agent at Webster & Co, to "be careful not to get any of the religious matter in" to the sales prospectus for Connecticut Yankee. But there's no evidence that he ever was tempted to delete or tone down Hank's comments on the Church, and he seems not to have objected to any of Beard's explicit illustrations of the novel's anti-Catholic theme. Hank's story has several different antagonists -- Merlin and Morgan le Fay, for example, and the titled aristocrats of the 6th century -- but throughout the novel "the Church" is presented as the greatest enemy of his project to enlighten the Dark Ages, and MT's emphasis on "the Church" as the most sinister force in Arthurian England goes way beyond the place it occupies in his medievalist sources -- Malory, Scott, Tennyson and so on.
Indeed, MT ultimately makes "the Church" the evil that prevails over Hank's Americanized Camelot, as the priests plot against him in secret and then push their followers into war against his republic. There's no Arthurian precedent for that plot development, but it does mirror fears about Catholicism that were widely shared in MT's America. Anti-Papism, of course, goes way back in American culture -- all the way to the Puritans who founded the country on Plymouth Rock and a hatred of Romanism. During the 19th century there were two great waves of anti-Catholic agitation: the Know Nothing era of the 1840s and 1850s was the first; the second was just reaching its height when Connecticut Yankee appeared.
As in the ante bellum period, increased immigration from Catholic countries helped precipitate the anti-Catholic crusade. Between 1860 and 1890, the Catholic population of the United States tripled (from about 3,100,000 to about 8,900,000), and according to many estimates the Catholic Church was the fastest growing demonination in the country. At bottom the motive of the bigots may have been economic -- the country's weak economy, and especially the Panic of 1893, led to fears about losing jobs to this new "foreign" element -- but their rhetoric stressed instead the idea that Catholicism was both inherently unAmerican and, as one writer put it in 1889, "on the make." The idea that the priests were plotting against the republic pre-existed MT's fantasy about Hank, and became stronger during the 1890s.
This section of the archive gathers selected examples of anti-Catholic rhetoric from the years immediately before and after Connecticut Yankee came out. In the material linked below, you can hear one side of the conversation that popular American culture was having about the relationship between Catholicism and American values. The voices you'll hear are angrier, more hateful and hysterical than Hank's, but they engage many of the same issues he raises in his critique of "the Church" as "an established slave-pen." How the novel's attitude toward Catholicism influenced its popularity with American readers is hard to say. MT himself asked one reviewer to avoid any mention of the book's "slurs at the church." Only one reviewer of MT's novel, a hostile critic in Boston, notices this element in its story, and he protests strongly, as a Protestant himself, against the "Protestant intolerance" he found in the text and, especially, the illustrations. Whether America's larger silence on the issue implies consent to Hank's fear of "the Church" remains an open question.