Gnosticism is an umbrella term for a variety of sects that may have predated Christianity (the exact origin, and whether it was “Western” or “Eastern”, is hotly disputed) but arose as a coherent religious movement in opposition to early Christianity, and largely faded away within a few centuries. As an esoteric (and often elitist) religion Gnosticism is connected to polytheistic mystery religions and to mystical forms of both Abrahamic monotheism and dualism (Manichaeism, Zoroastrianism, etc.)
While there were many people who considered themselves Christian Gnostics (and some who still do!), the Church has always considered Gnosticism heretical, especially because it positions direct experiential knowledge of the divine (insight, enlightenment) as the key to salvation from material existence. Gnosticism and Christianity share the idea of an imperfect world caused by a “fall from grace”, but where Christianity places the blame on sinful humans, Gnosticism puts it on a flawed divine being. While there are some pretty weird things going on in every Gnostic tradition, it is an interesting twist to have humans bear no ultimate responsibility for their own suffering.
Gnostics consider the entire material world to be at least imperfect and corrupted or even evil. There was never a Garden, a perfect expression of the material world. There is a singular, supreme God/monad who does not “create” anything but rather emanates similarly immaterial beings called æons, which only descend to the material plane through a mistake, flaw, sin. While in one sense everything is an emanation of God, the plane of existence that we live on is so twisted that we can consider it completely separate from the ideal, purely spiritual and immaterial world of Pleroma (Fullness). God is then transcendent rather than immanent, and it would be a mistake to worship nature or other deities contained within this world.
The creator of the material world is called the demiurge (a term borrowed from Plato, I think), who may mistakenly believe that he is God and have servants called archons (roughly analogous to angels or demons, and linked to specific celestial bodies). Gnostics have spent a lot of time making detailed classifications of æons and archons. Æons often come in male-female pairs called syzygies–Jesus and Sophia [okay maybe I had an ulterior motive for choosing this topic] are sometimes the lowest-level syzygy, most connected to the material plane. Other times Sophia is the æon which emanated without a male partner, resulting in the demiurge.
Given its conception of the entire material world as fundamentally corrupt, Gnosticism generally does not offer rules for moral or ethical conduct, leaving this to the individual to decide for themself. Still, many Gnostics looked to Seth (the 3rd son of Adam), Jesus, or Mani as savior figures. Ultimately, release from physical existence could only be achieved after death (and reincarnation would often be the consequence of not acquiring enough insight during one’s life), by overcoming physical existence, somehow undoing the mistake of creation, and returning to the Godhead (Spirit, Fullness, Profundity).
In 1945 actual Gnostic texts were discovered in Egypt; these and other texts which have since been discovered include many additional and alternative narratives to the New Testament. One can imagine why the Church, seeking to assert a single authoritative version of Biblical myths, was so threatened by the proliferation of alternate texts, ideas, and cosmologies that sought to operate both within and without the framework of existing religions.
Sources:
“Gnosticism.” Early Christian Writings. N.p., n.d. Web. 23 Feb. 2016. <http://www.earlychristianwritings.com/info/gnosticism-cathen.html>.
“Gnosticism.” Early Christian Writings. N.p., n.d. Web. 23 Feb. 2016. <http://www.earlychristianwritings.com/info/gnosticism-wace.html>.
“Gnosticism.” Theopedia. N.p., n.d. Web. 23 Feb. 2016. <http://www.theopedia.com/gnosticism>.Hoeller, Stephan A. “The Gnostic World View: A Brief Summary of Gnosticism.” The Gnosis Archive. N.p., n.d. Web. 23 Feb. 2016. <http://gnosis.org/gnintro.htm>.
“What Is Christian Gnosticism?” GotQuestions.org. N.p., n.d. Web. 23 Feb. 2016. <http://www.gotquestions.org/Christian-gnosticism.html>.