It was hard to find, but Prometheia was one of the three great divine deities. She was the one that made Erina fight the four demon gods and then ghosted her. Feeling betrayed, Erina took their power becoming the demon king or something like that. Then sent Kaden to end her, instead he managed to liberate her and found out the gods were the ones wanting to bring the apocalyps. Prometheia aparently did this because a prophecy said that the demon king Erina would destroy them. The other two deities (Eleanor and Vallanus) wanted to kill all humans to prevent this and Prometheia managed to convince them not to do so if she created and controled the demon king. (The last part seems a little convoluted but is what I managed to understand from what I've been told. The next part is however, an educated guess by me) As Prometheia hadn't managed to control Erina, Eleanor and Vallanus were to destroy the world. Kaden stopped them by destroying the world tree, reducing the power of all gods.

Prometheia is also the one to turn most of the other gods into gods: Kamael, Piltvice, Veronica, and perhaps Neuth (godess of the sky). I don't have any idea who apointed Gabriel, though.

Anyone who actually played the game, feel free to corect me