Heretics are they, who maintain stubbornly against the plain light of truth, any doctrine which overthrows directly, or by necessary consequence the fundamentals of Christian faith. The Rules. I. Not every Error makes an Heretic. For either there is an error against the foundation; as that of the Arians and Marcionites, of whom, they denied the divinity of Christ, these his humanity: ... For more, see Johannes Wollebius on the definition of heresy.