@bythos
as yahee explained, Demons are empowered by the Demon god, and the humans/good guys are empowered by the Goddess. It works similarly to the standard 'Dark' vs 'Holy', where Dark is incredibly strong but also weak against Holy. This is how the 'Good Guys' haven't been overrun- their magic system itself is tuned to Demon-killing.
The MC, on the other hand, isn't beholden to this, for better
and worse. He doesn't need to go through the complex steps that the Goddess requires, allowing him to be much more advanced in magic techniques, but at the same time doesn't get the 'blessing' from the Goddess, making his attacks far less effective against the 'Dark' type demons compared to weaker, but 'Holy' type element-based attacks.