Scythe Curie writes that the only natural death left is death by fire. She finds it comforting that there's one thing that the Thunderhead can't control, though it tries. It monitors smoke and heat everywhere. The Tonists sometimes try to burn their deadish, but they're often unsuccessful.
Because the Thunderhead cannot control fire, it stands to reason that fire is the last truly dangerous natural disaster. The Tonists that Curie mentions show that it's possible to try to harness fire as a tool, but it's not an easy thing to do.
Citra spends her days training and gleaning with Curie. She learns to recognize the signs of "stagnation," though she questions whether it's a good metric to go by. Curie does explain that she never gleans children. Scythe Faraday seldom gleaned children, but the one he gleaned while Citra was with him shook him. Citra vows to never glean children. She thinks sometimes of Rowan and hopes that if she can prove that Faraday was murdered, she won't have to glean or be gleaned by Rowan.
Notice how, as Citra progresses through her training, she begins to think more critically about the kind of scythe she'd like to be and idolizes her mentors' methods less. This shows her coming of age and beginning to fit herself into the Scythedom as a whole, while also showing the reader that between Citra and Rowan, Citra is the only one still honestly preparing to be a scythe.
Curie leaves reaching out to the families of the gleaned to Citra. Citra discovers that this is so she can develop compassion. One afternoon, Citra can only discover one estranged brother of Curie's latest gleaning, which is unusual. She discovers that the brother lives in a Tonist monastery. Normally Citra wouldn't go anywhere near Tonists, but she can't back out of tracking down family members. She lets herself into the chapel and tries not to look at the disturbing Tonist images in the stained glass. She doesn't know exactly what the Tonists believe, but she knows it's a silly conglomeration of mortal age faiths.
Curie has already said that the Thunderhead has rendered religion pointless in her society. It's worth noting that religion as a whole is intended to give believers a sense of purpose and a roadmap for how to live their lives, no matter who or what a person worships. This suggests that despite Citra's derision, there is still a reason for religion, it's just less pressing given that there's little incentive to make the most out of one's life.
Citra approaches a clergyman in the chapel. He introduces himself as Curate Beauregard and grudgingly agrees to fetch Brother Ferguson. Citra looks at the giant tuning fork rising out of a basin of smelly water, the symbol of the Tonists. Brother Ferguson appears behind Citra and tells her that she can strike the bident if she wants. She ignores this and tells him that his sister was gleaned, that he needs to arrange for a funeral, and that he's eligible for immunity. Maddeningly, Brother Ferguson says that death by scythe isn't natural, so Tonists don't acknowledge it. He doesn't want immunity and will let the Thunderhead provide a funeral.
Though Brother Ferguson tells Citra little about what the Tonists actually believe, it's important to note that he doesn't accept the role of scythes in society. In particular, his insistence that a scythe's gleaning isn't natural suggests that the Tonists aren't actually all that sold on the idea of immortality at all—meaning that they might actually be strange allies for the Scythedom, who seek to make sure that death still happens in their immortal world.
Angrily, Citra picks up a mallet and hits the bident as hard as she can. It resonates loudly and is simultaneously jarring and soothing. Brother Ferguson says it was G-sharp, though some Tonists believe it's A-flat. He offers to answer any of Citra's questions. She asks what Tonists believe, and he says they believe "flames were not meant to burn forever." He says they don't actually worship darkness; instead, they worship "wavelengths and vibrations" that humans can't see. They believe doing so will “free [people] from being stagnant.” Citra asks about the dirty water, and, excitedly, Brother Ferguson says it contains disease from the mortal age—but now, it does nothing to people.
Brother Ferguson confirms here that the Tonists don't believe that immortality is a good thing, though the rest of his explanation is much less convincing. The pool of diseased water suggests that the Tonists really worship the Age of Mortality and the way that people lived then, and so they try to mimic the Age of Mortality lifestyle as much as they can. Again, this seems to line up with some of the Scythedom's beliefs.