Dean Winchester: Jack brought you back because we needed you back.

Castiel: Right. And how have I repaid him? I promised his mother that I would protect him, but now he’s trapped in that place while Lucifer is here, who’s… I mean, he’s getting stronger and more powerful by the day. And if Michael really is coming, maybe I was brought back to help prepare.

Dean Winchester: Prepare for what?

Castiel: War. War is what Michael does.

Dean Winchester: Well, then we do what we do. Whatever it takes.

From Supernatural – Season 13 Episode 14: ‘Good Intentions’ (13×14)