Is Gnosticism a dualistic religion? Is dualism a vital tenet of what makes a person or a sect Gnostic? My short answer is (despite what the heresiologists have always claimed): No.
Firstly we need to address what dualism is in relation to theology. Dictionary.com states that dualism is “the doctrine that there are two independent divine beings or eternal principles, one good and the other evil” and/or “the belief that a human being embodies two parts, as body and soul”. Since the latter was not as emphatically denounced by the heresiologists as the former notion of two opposing gods or principles, I will address the first one here and the second one in another post.
Many Gnostic sects, including the Sethians and Valentinians (who are usually considered the main influences of modern Gnosticism), ascribe to what can be called Emanation Cosmogeny, the theory that the universe and everything in it is “created” through a series of emanations, or, perhaps more accurately, is emanated, not created. This is readily supported by the vast number of “Aeons” (emanated principles or eternal beings) that are used to describe the creation process of the cosmos. Indeed, this is also supported by the Qabalah (or Kabbalah), a kind of Jewish mystical gnosticism of its own, where the spheres of the Sephiroth (which could easily be likened to Aeons) are emanated one following the other to make up the Tree of Life, the effective “map” of the Universe, and these in turn are emanated through the Three Veils of Negative Existence from the “Source” which the Gnostics would have (and do to this day) called the Pleroma (“Fullness”). Now, how can this notion of the “fullness” of God equate with the radical dualist notion of a good god versus a bad god?
To elucidate how different these two “theisms” are, check the following diagrams. The first shows the radical dualist concept (one that is, admittedly upheld by certain Gnostic groups like the Cathars and Manichaens, though sometimes in a more mitigated form). The second shows panentheism (not the same as pantheism), which is most accurate for the Gnostic conception:
The Qabalah also has a concept called “Tzimtzum” (“contraction” or “constriction”) which relates to this Emanation Cosmogeny. The main element of this concept is that God “contracted” himself in order to grant the space for a finite world to exist in. to conceal from created beings the activating force within them, enabling them to exist as tangible entities, instead of being utterly nullified within their source”. It also contains the paradox that God is simultaneously transcendent and immanent.
To end, Father Jordan Stratford+ of the AJC has an excellent post on the cosmogony of Gnosticism, part of which says: “Gnostic Tradition teaches that the Pleroma ("the Fullness") is the Ultimate Godhead; everything – everything – radiates out concentrically from the Godhead like ripples from a stone dropped in water: Christ, Sophia, the Demiurge, you, me, chartered accountants, loofas, squids, ginko trees. The Pleroma is also the stone, and the water, and the idea and act of "dropping". This is a good analogy, as a wave/ripple is a transitory expression of a phenomenon rather than an object unto itself.”
[Images based on the designs of Fr. Jordan Stratford+ and Jesse Folks' "theism" images]