Capítulo 412: Play Time Is Over.

God fire is the very flame created from a god spark when someone becomes a god. It burns almost everything apart from the god that it belongs to. Even a vessel that is attuned to that god is not safe from it. A god needs it to convert faith to divine energy so it is very important to a god. It can be said to be the foundation of their faith.

Using the god fire on things apart from faith energy will deplete it until it dies off because only faith can fuel it. Using it on a god's vessel will quicken the rate at which the vessel deteriorates and dies. It will also weaken the god. That's why the Celestial was holding back on using his full power from the start.