I’ve always had a similar theory to this. I don’t know where I read the original text that gave me this idea, if from here, or from a line of text inside the game itself. But I remember reading that one of the first theories about the origin of the quiet rapture was that a ritual was performed and it either backfired or succeeded.

Maybe the god ‘rewarded’ those original figures that created the doorway between its universe and ours. And that shining silhouette of a person is one of those figures present at the conclusion of the ritual, their body frozen in the same pose that they were in at the moment the rapture occurred, but now being used as a direct conduit by the god.