Curiously, it's heavily implied in his exchange with Mephisto that spoilers:end of spoilers I swear to God, if Satan ends up becoming Galactus or Unicron...
Rin witnessed or recalls his probable future as Akumasai the Magical Girl Slayer, which will lead to some interesting character development. And it's revealed that Satan is draining life energy from humans and high-level demon alike as his "territory" expands in his process of creating a new body for himself, which effectively makes him a threat to everyone.