He nods again, then focuses all his attention on Meg and the thing inside her.

"Regna terrae, cantate Deo; psallite Domino."

Kingdoms of the earth, sing unto God; sing praises to the Lord.

There's conviction in his tone, itself speaking of an underlying belief in the power behind the ritual. As he speaks, Sam begins to walk clockwise, circling Meg.

"...qui fertis super caelum caeli ... "

... that carry above the sky of heaven...

Meg doesn't try to twist around to look at Sam, but stares at Dean in something approaching disbelief.

"An exorcism? Are you serious?"