The term (from the Greek gnosis, "knowledge") refers to a loosely defined group of religious sects which flourished near the beginning of the Christian era. They were all syncretistic, incorporating elaborate myths, elements of Hellenistic mystery cults, Greek philosophy and mythology, and features of Christian and Jewish faith. Some gnostic teachers regarded themselves as Christians, but theologians like Irenaeus and Tertullian denounced them as heretical. They were called gnostics because they consistently understood salvation as a deliverance from the material world and held that salvation came through knowledge of "otherworldly things." This knowledge was usually secret.
The features which most gnostic systems share are 1) spirit-matter dualism, with matter regarded as evil; 2) a creation understood as botched, not effected by the supreme God but by a lower "demiurge"; and 3) the existence of an elite group, the "spirituals" (pneumatikoi), who possess a divine spark which longs to be released from the body and return to heaven. This deliverance was to be accomplished through the gnosis offered by the sect.
The NT insistence on the goodness of creation and salvation by the Cross alone stands as the bulwark of Christian orthodoxy against gnosticism.