From the Soundless cult's perspective, any human who had witnessed the domain of God and the world where everything was living would find themselves being shone on by the light of God, thus understanding the truth that the world was silent.
If a normal person were to be put into that situation, even if they weren't fully assimilated into it, their perception would be altered from witnessing the unseen aspect of the world, which would eventually manifest as an irreversible mutation.
Furthermore, an individual's perception must collapse to a certain degree before they could recognize the silent power in their surroundings, thus dispelling it and returning from the other aspect of the world.
Such people would then be easily inducted into the Soundless cult, because a portion of their perception had already been broken and made them feel more attracted to the aspect where everything was alive rather than this one.
