Quiz

Answer: Fingerprints.

Correct: Papillary lines.

Ugh, how beautiful!
But I must say that, yes, the lack of a normal real-time discussion makes the task quite difficult. It's impossible to have a proper brainstorming session in a forum format.

Next, the priest demonstrates a miracle, which, for greater persuasiveness, could involve raising his hands to the sky for a second.

An association with Moses. Parting the sea or conjuring mana from the sky, something like that. However, there should be some kind of semantic sequence here – first, X (masculine gender, right, Toxic? Or is it not necessarily?), then this miracle. Before mana, in terms of semantic sequence, there could be water from a stone (which would fit, like a water pipe or something similar, but conjuring mana from the sky is a difficult task). Before parting the water, there could be the plagues of Egypt (but it's unlikely that mass killings of African firstborns or locusts specifically targeting Africa occurred in the twentieth century).
Maybe I'm still wrong, and it's not about that... Another idea that comes to mind is military victories (there's a direct connection with raising hands to the sky, if we remember Moses again), but something doesn't quite fit.
Another silly option: the natives were shown electricity, for example, and they fall from the shock (but that's too cruel). Then the miracle that follows is the "resurrection" of the unfortunate shocked natives (shock them with electricity again?).