@Kggg imagine if the strongest being in the world had a child with the strongest anti-being. People would feel highly threatened and likely target them and their child, yes they could probably take anyone who comes after them, but it still puts their child at risk, and it's not a good environment for raising the child.
@Granitefish
you also need to add that as a child of that "anti-being", gospel have strong resistance or even immunity to his ability, aka "anti-demon gas". So they feel even more threatened knowing that their strongest weapon literally have no effect on monster that can very well be "disaster class".
And if warmonger demon faction would know this, they also can try to kidnap him, to "rally under him" against humans, since he will definitely would be ultimate weapon against them, if raised "right".