Gnosticism

Gnosticism (after gnôsis, the Greek word for "knowledge" or "insight") is the name given to a loosely organized religious and philosophical movement that flourished in the first and second centuries A.D. Though the exact origins of this school of thought cannot be traced with any certainty, it is possible to infer that certain non-canonical sources dating from the first and second centuries A.D. were used to develop it.

There are numerous references to the Gnostics in second century proto-orthodox literature. Most of what we know about them is from the polemic thrown at them by the early Church Fathers. Marcion was the most famous of the Gnostics, and he established a "canon" of the Pauline epistles that did not include the pastorals and a "mutilated" Luke. Justin Martyr mentioned Marcion c. 150 CE, and Irenaeus and Tertullian wrote against him extensively in the late second century (in Against Heresy and Against Marcion, respectively).