"We don't need prophets from four hundred years ago to know what they're planning," Crowley says. "We know them. We worked for them. We sent them memos, we followed their moves. They...they just do what they think is the cleverest. And we were cleverer than them for six thousand years, angel. We can beat them now."

Can and must are interchangeable here, because they have to be. Crowley can't see another way they're getting out of this one.