That quote from slan is the sole reason why I started this whole conversation about Guts using a beherit
And it's something that is very often discussed. Here's something I wrote about it recently:
I don't think the series has ruled out a sacrifice using a Beherit. It's just unlikely to happen in the story, and wouldn't make a terrible amount of sense. For years I thought it strictly wasn't possible, because of Conrad's words in Vol 3. But then I noticed something about
when God Hand talk are talking about Beherits and being ordained by causality.
I think what Conrad said had less to do with Guts being a sacrifice, and more to do with his relationship with the Beherit. Conrad said Guts wasn't "ordained" to be among them as an apostle during the Count's ceremony. But shortly afterwards, Guts ends up with a Beherit, which he's carried around ever since. Now, let's recall Ubik's words to Griffith in Volume 12: "From the moment you took possession of that crimson Beherit, you had the qualities to become a demon ... No, perhaps I should say that because you had those qualities, it fell into your hands."
Though these two dialogues are separated by almost 10 volumes worth of material, the thrust of this argument is reinforced by the repeated urgings from Slan (Vol 3, 26) and the specters (Vol 16), for Guts to use it. That could all be merely dramatic tension or a true threat. But if he ever took the offer, it would be a dead-end in terms of the story.
Let's apply that potential outcome to the two most obvious scenarios:
Guts takes the deal, however unlikely that sounds. This effectively ends the series. Guts is no longer a human, and the God Hand have influence over him, just like what happened when Ganishka faced Griffith in Vritannis. Thus, the conflict at the heart of the series would come to a swift and unsatisfying end.
Casca takes the deal. There'd be no saving her from that. Whether he can bring himself to kill her or not, Guts would be demoralized, and his consistent savior to relinquish the Beast's hold on him would be gone.
So... yeah, it's not something I like spending too much time debating. I'm a pragmatic guy, and hypotheticals aren't really my thing. Instead, as we've said here for years, I can't help but think that Miura has bigger plans for it than merely another apostle ceremony.
The God Hand would presumably have less power over controlling the causal web of existence after the Blast. Material reality is the most objective layer of existence in which substance denies desire—you can stare at a door all day willing it to open but it won't budge. But the higher you go on the layers the more substance conforms to desire and the less objective reality is. So the God Hand would be more powerful in a micro sense, but they would be less powerful in a macro sense by having less power over all causality.
That may have made some sense before the Blast. But after? We don't know the current makeup of the world. It's a huge variable. Besides, "controlling the causal web of existence" is not exactly what the God Hand do. We've seen them in action behind the scenes. They pull the strings on various large-scale world events (Conrad and the plague, Slan in the presence of the cultists, Femto likely responsible for the mass dream). The deeper level manipulations that I think you are referring to are part of the Idea of Evil's purview.
