Gnosticism is a collection of religious ideas and systems that coalesced in the late 1st century AD among Jewish and early Christian sects. These various groups emphasized personal spiritual knowledge (gnosis) above the proto-orthodox teachings, traditions, and authority of religious institutions.

Gnosticism (pronounced NOS tuh siz um) was a second-century religious movement claiming that salvation could be gained through a special form of secret knowledge. Early Christian church fathers such as Origen, Tertullian, Justin Martyr and Eusebius of Caesarea condemned gnostic teachers and beliefs as heretical.

The term, based on the Greek gnōsis (“secret knowledge”), was coined in the 17th century, when it was applied liberally to ancient Christian heretical sects, especially those described by their orthodox contemporaries as radically dualistic and world-denying, and those who sought salvation through esoteric revelation and mystical spirituality.

" Gnosis " is the ancient Greek word for "knowledge", but now more closely means " esoteric knowledge". Gnostic ideas were a main part of early Christianity before being persecuted by other Christians. Gnostic ideas are also found in sects of Judaism and Islam, as well as in alchemy and other kinds of esotericism.

Gnosticism in modern times (or Neo-Gnosticism) includes a variety of contemporary religious movements, stemming from Gnostic ideas and systems from ancient Roman society. Gnosticism is an ancient name for a variety of religious ideas and systems, originating in Jewish-Christian milieux in the first and second century CE.