[[File:Irenaeus.jpg|thumbnail|right|250px|In 180 A.D. [[Irenaeus]] wrote ''Against Heresies'' which is a [[Christian apologetics|Christian apologetic]] work addressing the heresy of gnosticism. ]]
'''Gnosticism''' (after ''gnôsis'', the [[Greek]] word for "knowledge" or "insight") is the name given to a loosely organized [[religious]] and [[philosophy|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. It had been considered a [[heresy]] by [[church fathers]] and so is by most modern Christians. Most Gnostic writings are exceedingly obscure and tedious.<ref name="Smith"/>