The dead in Christ could be a few things. Believers that return to fruitful ways, memorializing believers that have passed on, or a sort of collective wisdom that goes into the future that is a compilation of humanity-saving testimonies that help heal and unite. Neat to dissect the symbolism.
Imagine interactive museums with famous figures acted by AI. You could walk into an immersing experience and request to have a conversation with anyone from history. The AI would algorithmically provide their strongest arguments for any question asked based on whatever writings they left behind.
You're on the right path, though. When you see the magnificence of it, you realize it was collective humanity stuck in a thought process loop of circular belief systems.
That's some really fucking wild ass shit, man.