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 Albigensians
Offers an account of a Catholic Inquisitor on the Cathar faith.

Strange Fire: Rise of Gnosticism in the Church
Book by Travers and Jewel van der Merwe offering a Christian view of ancient and neo-Gnostic beliefs.