Catholic Encyclopedia: Gnosticism
Gnosticism, as a syncretistic, dualistic system has been historically regarded by orthodox Christians as a heresy.
Definition of Gnosticism
Gnosticism died not by chance, but because it lacked vital power within itself; and no amount of theosophistic literature can give life to that which perished from intrinsic and essential defects.
Medieval Sourcebook: Bernard Gui on the Albigensia
Offers an account of a Catholic Inquisitor on the Cathar faith.