Definition: Gnosticism comes from the Greek word gnosis 'knowledge'. Gnostics believe they have knowledge about God through which they may attain salvation. In the early Church, Gnosticism was one of the main types of Christian belief systems. It flourished in the first century. Later, Gnosticism was considered a Christian heresy. Gnostics believed that there was an evil God of this world that was different from the abstract God presented by Jesus Christ. This is gnosticism's characteristic "dualism".
Irenaeus and Tertullian wrote against Gnosticism. Marcion was the most famous Gnostic of the Early Church, although he may have simply been a dualist and not also a Gnostic.
Sources: Gnostics, Gnostic Gospels, & Gnosticism
IEP Gnostic
