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.