Deeply Rooted.

Rasmus brought the casket away from the pond and Videl devoured all the trapped spirits inside it. If he hadn't devoured the demon inside Guile, he wouldn't have been able to devour those spirits. The trip into the forest was the perfect opportunity for him to regain his power.

Videl explained that divine power was indeed powerful against demons, but humans had a limit on how much divinity they could use. As the demons grew stronger, divine power wouldn't affect him as much as before. If divine power was the water, demonic energy was the fire, and a matchstick would get extinguished by a single droplet, but it wouldn't be able to extinguish a torch.

Since humans had limitations, they couldn't do anything to powerful demons like Videl or Satan. However, demons of lower than Sovereign rank could lose half of their power the moment they came down to the world of the living to maintain their existence.