The One Who Waits gives the Lamb a choice to become their vessel and come back to life. But it’s a classic case of But Thou Must!. Not only because the Lamb dies otherwise but there are cases in real life where indoctrination is forced upon someone.
There's no practical benefit at the end of the game for allowing yourself to be sacrificed to The One Who Waits for the Bad Ending. But the fact that the choice exists shows that the Lamb, unlike the beginning, is now capable of deciding their own fate. This is not a case of But Thou Musn't; they stand up to fight back because they want to, not because the game railroads them into it.
The customization of followers is just like how real life cultists would give up their identities for a new one after joining.
Marrying multiple followers will cause them to become jealous of each other. While annoying to deal with, it does make a good deal of sense - even though they're all aware of the Lamb's importance and how they can basically do what they want in their campaign to release The One Who Waits, they still have their own thoughts and feelings that will be affected, unless the Lamb decides to brainwash them of course.
Murdering a follower will cause other followers (if they witness the deed) to become very upset and your Faith to take a serious hit. Apart from the obvious shock of watching someone die, it also reveals how much they can tolerate - they'll be kind of upset if the Lamb performs a Sacrifice or harvest a dead follower's body for resources, but reassure themselves that it's for a reason. Killing a follower in broad daylight, though? It can very well fly in the face of the Lamb's benevolent tendencies and make it much harder to worship them. If they're willing to kill you for no apparent reason, what kind of leader are they, really?
Killing/sacrificing dissenting followers being the exception to this, since if they're blaspheming the Lamb, then punishment is clearly deserved.
Allowing Twitch viewers to raffle themselves into a streamer's cult aligns with how a streamer's community can develop an oddly dedicated parasocial relationship sort of like a real cult.
Shamura is associated not only with war, but also with knowledge. Unlike chaos, famine, or plagues, war requires anyone going about it to be smart if it's to cause a lot of deaths.
Cultists seem to age a year for every day they live in your base. Perhaps that's the reason life is so cheap in this world — when your life expectancy isn't more than two months, an early death isn't that big of a loss.
There are a few NPCs that can last forever. Of them, the Fisherman, Plimbo, Helob, Rakshasa, and Forneus are the only ones without some explanation for their agelessness. Ratau presumably still has some power left over from his time as the One Who Waits' vessel, extending this to his Knucklebones buddies, Ratoo is presumably undead or just can't die without his heart, Claunek and Kudaai make it clear that they're immortal, Haro knows more than anybody else and may also be immortal, the Axolotls are non-specific enough such that you may not meet the same ones every time, Sozo is most likely a zombie possessed by his mushrooms by the time you run into him, Midas has magic and may be immortal and the Fox is clearly not your average fox and is likely immortal.
It's also possible that anyone who worships the Bishop of Death (without being a vessel) has their natural lifespan shortened, which explains some the NPCs above are ageless — it's not that they're immortal, it's just that the Lamb's followers traded most of their lives away.
On the One Who Waits being a cat:
Kallamar is stated to be the Bishop most afraid of The One Who Waits. Well, of course. Kallamar is a sea creature and The One Who Waits is a cat.
The Offering Chest has varying prices for each item sold to The One Who Waits. Most items sell for 10 gold or less, but the rarer fish sell for 20 gold. After all, cats do love their fish.
Of course the One Who Waits rebelled against his siblings — cats are well-known for doing what they want.
The Bishops of the Old Faith are stated to represent the Four Horsemen of the Apocalypse, with Leshy being the only odd one out with no corresponding horseman. Yet looking at the passage containing their descriptions, there is one who accompanies the four horsemen that Leshy could correspond to and that's the Hell that followed after them all. Considering Leshy represents Chaos and is considered the youngest of the Bishops, it makes sense that he would represent a very chaotic place and the last, aka youngest, who followed the Four Horsemen.
Rather than each of the Bishops representing one of the Horsemen, they may instead be facets of the fourth Horseman, Death. The fourth Horseman is said to have the power to kill with sword (Shamura), pestilence (Kallamar), famine (Heket), and wild beasts (Leshy).
Shamura and the One Who Waits seemed to have been very close before his imprisonment. This makes sense when looking at their aspects — while pestilence, famine, and chaos either happens naturally or can be cultivated, war must be planned and sought out. As seen throughout the game, the One Who Waits prefers people who actively causes death around them. As for Shamura, death is essential to defining war, both in its strategies and its tragedies.
None of the cult's symbols actually involve the One Who Waits. The iconography is either centered on the Lamb themselves or on the Red Crown that provides them power. This might go a long way to explaining how the Lamb is even able to usurp The One Who Waits to begin with - sure the Lamb was being entirely truthful verbally, but how many followers would put their faith towards a god they don't even know the appearance of, versus his prophet who wields the symbol of his divine power.
Leshy was blinded by the One Who Waits before his imprisonment, which is reflected in his boss battle; his attacks are randomly spawning enemies about the battlefield, a set spike-ripple attack, a set of melee slams around him and a volley of randomly positioned shots. None of his attacks are actually very accurate, merely hoping the Lamb runs into them or is distracted such that his minions can get the drop on them. The best he has is the projectiles, which are aimed, and even then he only shoots those in the Lamb's general direction, possibly shooting in the direction of the noise the Lamb makes when running around.
Repeatedly hurting the giant snail behind Rakshasa will eventually cause him to attack the Lamb. After being defeated, Rakshasa will give the Lamb a blueprint for the Massive Monster shrine. Almost as if he's calling the Lamb (and the player, to an extent) a massive monster for daring to attack his wife...
It can seem odd that a Follower can ask you to sacrifice another "as a prank", which is... rather ruthless for a prank. Are they not as innocent as they seem? Or are they just Genre Savvy and know you can just bring them back whenever you want? And that if you're the kind of person who takes issue with their making this request, you probably will bring them back as soon as possible.
Speaking of resurrections, followers witnessing it — including ones who specifically ask for you to do so — probably lose faith because the cutesy art style obscures what a messy affair resurrection actually is. Even with what we do see, the resurrected follower's corpse makes a horrible gurgling noise before life suddenly pops back into them. In short: They don't like doing it. It's not a pretty sight.
You unlock the Wolf follower form by doing one of the most evil actions the game lets you do — sacrificing a harmless old man who's done nothing but be kind to you and practically sees you as if you were his own child — because by doing so, you've proven without a doubt that's what you are: a Wolf In (literal) Sheep's Clothing.
Sozo appears to be an ant, which combined with his love of mushrooms, appears to be a reference to the zombie-ant fungus, a fungus that similarly infects ants and grows from their heads.
Why do the postgame doors render the follower you have to offer to them Killed Off for Real? Because the reason for the crusade is for the Lamb to come into their role as a newfound god of death. While they can flaunt the laws of nature in their cult and do it, they're having to act on behalf of the cosmic balance and their new domain of godhood, it would run counter to that for the Followers to be able to come back to life.
The easiest way to Damn a follower with their sin (by making them generate a Sin resource 3 times in a single day) is with the Sinner's Pride ritual which makes them take on the sin of every other cultist. Of the seven sins, Pride is considered to be the deadliest, so it makes sense that would be the easiest one to push them past the brink, as opposed to just drinking alcohol, partaking of pagan dance or showing off their body in an act of lust.
The game justifies its item selling mechanic by making it an offering to The One Who Waits. So who gets the items in the postgame? After he's defeated, TOWW is either dead or in your service, but the offering chest is unaffected.
After defeating The One Who Waits but before encountering ???, the shrine produces gold instead of devotion. Perhaps the offering chest’s gold comes from the same place?
The Lamb regularly cries Tears of Blood when manifesting The One Who Waits' powers or conducting rituals. It's not clear if it's just a visual effect, or if they're actually bleeding from their eyes. If it's the latter, then they're surely going to experience some eye-related problems after a certain point. Crying blood so many times can't be good for their health, especially if they manage to make significant progress without dying at any point.
The Lamb is immortal, so they probably no longer have to worry about their health.
Natural Burial allows you to convert corpses into Fertilizer. Aside from the mild horror of knowing that your followers are eating crops grown from the remains of their fellow cultists, this also means that it's entirely possible their homes are also made of those cultists.
The ability to do this is unlocked by declaring a doctrine ("Return to the Earth"), which probably means they're well aware of it but are taught that it's the right thing to do.
It isn't just enough that that Heket's style of sacrifice is a disturbing way to go but realizing that the victim being slowly consumed by the fungus means the victim is being used as fertilizer.
The lighthouse keepers mentioned having a leader who disappeared after she went for a walk near the pier at night. At night when you go near the pier for the first time, you'll find the Fox, who is very hungry...
Chemach is surrounded by "followers" that appear to be haphazardly thrown together dummies. The Lamb can attack and destroy them (which Chemach will fret over but not do anything about). A sign that something's very wrong, though, is that they may drop Follower Meat when destroyed — one would easily assume this means Chemach made them out of corpses. However, if wearing the Golden Fleece, you can find that destroying them adds to the Lamb's Kill Streak, powering up the Fleece's effect. Whatever Chemach did to them, they're still alive.
, a fungus that similarly infects ants and grows from their heads.