This setting follows a religion in which god, originally a perfect entity and the seat of human consciousness, split himself into two separate beings and reincarnated into two mortal forms. These two beings would be the primogenitors of the human race, each possessing half of god's original qualities and attributes and explaining the sexual dimorphism in our species. We are, in a sense, all demigods due to having a smaller piece of our deity in each of us.
Only God can create new life, and both man and woman possess a metaphysical key inside them that can access that ability. When those separate keys unite, they can unlock the gate to where that power resides and access that potential. This is how new souls are created and brought into the world. Since each new human being possess as piece of God inside them, it stands to reason that God is getting weaker as more souls are being born. Why would this not be the case?