Gnosticism is a diverse and complex spiritual-philosophical movement that first emerged in the Greco-Roman world during the early centuries of the Christian Era, specifically in the 2nd century. Coined from the Greek root-word "gnosis," which translates to "knowledge" or "insight," Gnosticism is a broad reference to a select group of people in pursuit of advanced spiritual knowledge.
With a focus well beyond attaining mere ‘book knowledge’ i.e. academic or theoretical knowledge, the gnostics sought out a very special type of knowledge. That which was directly capable of fundamentally transforming one's soul, inducing an ‘alchemical’ reconfiguration of one’s very being. To be sure, the hidden truths they pursued were performative in nature, directly capable of triggering mystical experiences.
Now although Gnosticism may well be regarded by most as a major variant of Christianity – certainly a major heresy, as far as the early Church Fathers were concerned – it was not strictly confined at all to the essential Christian framework. Indeed, Gnosticism as a religious movement did not in fact possess its own body of sacred literature.
Rather than pose as a hindrance however, this fostered a unique approach towards building up a gnostic ‘body of work.’ The essential method being to ‘borrow’ or ‘mine’ the sacred texts of other cultural or religious traditions, including Christian, Jewish, Greek, or Oriental.
The overarching purpose being to extrapolate deeper, transformative meanings from such sources, allowing the gnostics to derive the mystical knowledge they ultimately sought. Even to claim such ‘borrowed texts’ as their own, on account of the added richness they drew out of them.
To learn more about Gnosticism and its aims, and how it differs in particular from Orthodox Christianity, one may consider the following additional information:
The Gnostic understanding of the ‘Garden of Eden’ story, and how it differs from the Orthodox Christian interpretation.
The Gnostic pursuit of ‘The Mysteries.’ That special magical knowledge capable of purifying one’s soul, and protecting against evil entities (Archons).
How the contrary doctrinal positions on salvation between The Church and the Gnostic movement led the former to persecute the latter.