"Fanatics may suppose, that dominion is founded on grace, and that saints alone inherit the earth; but the civil magistrate very justly puts these sublime theorists on the same footing with common robbers, and teaches them by the severest discipline, that a rule, which, in speculation, may seem the most advantageous to society, may yet be found, in practice, totally pernicious and destructive." -- David Hume

More of an anti-fanaticism quotation, but it seems to belong.