Joris-Karl Huysmans's Là-bas had been rather scandalous, in a literary way, and it associated Huysmans in a sensationalistic way with occultism and Satanism, an association that Huysmans did nothing to suppress. But a bigger scandal than being associated with the occult was on the way.
In writing Là-bas, Huysmans doesn't seem to have had any particular thought of extending Durtal's story beyond the encounter with Satanism, but he seems to have felt, in publishing it, that there was more that needed to be written. Some of this is perhaps structural. Là-bas leaves Durtal with a negative recognition -- that the rationalistic mythology of the self-image of the age is a lie, as seen in the occultists who are not, as the mythology suggests, relics of an older age (the way Carhaix's obsession with ringing church bells actually is) but created by the modern age itself -- but such a negative recognition raises a lot of questions about what better way of seeing things there might be. There are also reasons connected with Huysmans's increasing Decadent distaste for the timidity of Naturalism as an artistic movement; he had already done something to show that Naturalists failed even seriously to explore evil and suffering, and now he could show that it also failed seriously to explore penitence and mysticism and sanctity, a white book to follow his black book. And, of course, some of it may well have been psychological -- Là-bas had drawn heavily from Huysmans own psychological state and reflection at the time, and he himself had already been moving beyond where Durtal had been left at the end of that work.
In any case, he set out to write Là-haut (Up There) to complement Là-bas (Down There). This turned out to be quite difficult. He wanted a book that captured the aspects of human experience that we associate with mysticism, but when he attempted to work through this, he find himself dealing with a very different aspect of human experience that he found personally difficult to disentangle from mysticism: sex. One of the early working titles is The Carnal Battle, as it seems to have become heavily dominated by his struggle with sexual temptations and at the time. Further, Huysmans found that he was much less well equipped to explore perennial mysticism than he had been to explore modern occultism; the whole thing mired him in a massively greater amount of research. For someone with a Naturalist background, the research was not necessarily a problem, but it required learning a very different vocabulary and set of assumptions than necessary for, say, reading sociological studies.
What is more, he had difficulty pinning down key elements of his story. His original idea seems to have been to weave in the story of the Marian apparition at La Salette in 1843 (which was controversial even among Catholics, in something like the way Medjugore is today), but as the novel approached completion, he scrapped the entire thing, rewriting it again without anything to do with La Salette and trying to distance it even further from Là-bas. He also seems to have felt at times that it tended toward the artistically dull, and when he finally published it in 1895 under the title En Route, he expected it to be disliked by everyone, Catholics as well as freethinkers.
It was indeed scandalous; the sexual temptation theme from the Carnal Battle stage of writing was still on display, and shocked even some freethinking types with how explicit it was. But that was nothing compared to the scandal of conversion. The literary world could tolerate occultism as a sort of artistic eccentricity; it had no idea how to tolerate religious conversion as an artistic eccentricity. Indeed, pretty much everybody, freethinker or Catholic, had difficulty accepting that Huysmans really believed any of the religious material that made it into his novel. Everybody agreed it was very vividly written and everyone was also skeptical of the idea that it was in any way really Christian, rather than just mining Christian symbolism for literary effect. They also, despite liking the writing, found the book itself puzzling -- a novel that was not really a novel, brilliantly describing a short period of events in which nothing much actually seems to happen, as if someone were to write a 'novel' about an uneventful vacation. Perhaps the best way to summarize the novel is Huysmans's own summary:
The plot of the novel is as simple as it could be. I've taken the principal character of Là-bas, Durtal, had him converted, and sent him to a Trappist monastery. In studying his conversion, I've tried to trace the progress of a soul surprised by the gift of grace, and developing in an ecclesiastical atmosphere, to the accompaniment of mystical literature, liturgy, and plainchant, against a background of all that admirable art which the Church has created.
[Interview in Le Figaro, 5 January 1895, as quoted in the Introduction to J.-K. Huysmans, En Route, Brendan King, ed. and tr., Dedalus (Sawtry, UK: 2024), p. 15.]
