Are you saying that the author intended “it” to refer to “talk” and not “Christianity”? If so, that’s one doozy of a misstatement on the part of the author. If not, I don’t see where the statement is used improperly.
On the other hand, it’s a little more ambiguous whether “it” refers later to Christianity, or civilization, but then where is the ambiguity to “and the Devil could not invent a better engine to spread his work than the Christianity of the nineteenth century.”
Particularly striking is that the author uses no modifier on Christianity in that last sentence. Had he said, “so-called Christianity,” I would be able to charitably read into his statement that he was merely saying that there was a lot of evil going on under the guise of Christianity in the 18th century. But as worded, he seems to plainly be referring to such devil’s work as Christianity itself, not some perversion of it.