Although the origins of Gnostic thought are controversial, many of the core themes and terms undoubtedly stemmed from Greek philosophical thought, especially Platonism. That did not necessarily mean that early Gnostics were taking these ideas directly from Greek thinkers or schools, rather that they came from a Jewish (and emerging Christian) world that had long sought to integrate Platonic concepts. Any attempt to separate Greek and Jewish elements in this synthesis is doomed to failure.
In the Gnosticism of the second and third centuries AD, we trace many Platonic themes:
-The word “Gnostic” itself derives from Platonism, although not in anything like its later religious or esoteric sense. Rather, it suggested knowledge in the sense of talent or ability. Over time, though, Christians and other groups adopted it for their own purposes.
-Also from Platonism is the idea of the Demiurge. Although Gnosticism adopted the concept, though, its substance changed radically. In the Platonic tradition, the Demiurge was a benevolent being seeking to create the best possible world. Gnostics saw the Demiurge as a flawed being responsible for a defective material creation.
-Middle Platonists postulated a division between the highest Creator and the inferior world-soul linked to matter. There was an immovable First God, Nous or Mind, the One or the Good. Derived from him is a World-Soul or Demiurge, that was in motion, and therefore inferior. This is the being that creates and governs the world. That is very reminiscent of the Gnostic world-view, in which the material universe was created by an ignorant lesser deity, sometimes called Ialdabaoth.
-Both Neoplatonists and Gnostics shared ideas of emanation, that is, the process by which lower kinds of reality emanated from the godhead. As that flow travels further from the source, so it progressively loses its divinity. This idea was attributed, dubiously, to Plato.
-Neoplatonists and Gnostics also looked to Plato’s Republic for the idea of a contemplative ascent to the divine world.
-Platonism teaches the transient and illusory quality of the visible material world. While that concept becomes fundamental to Christianity, it had special force within Gnostic systems, which so often taught the need to free oneself of illusion. That lesson was often taught in metaphors of sleep and awaking. When it accepts the material world as real, the soul is in a state of sleep, from which the Redeemer awakes it. That idea gains support from many New Testament passages in which sleep plays such an important symbolic role.
-The word hypostasis is rooted in Platonism, meaning the underlying reality or substance of something.
-The process of Bible translation also played its part. From the third century, Jews had access to their scriptures in Greek translation, especially the Septuagint, which became immensely popular. As they read the sacred text, they found words and concepts that had particular resonance within the contemporary philosophical and Platonic framework, not least words like gnosis itself. In Genesis 1, Philo read that God made man “according to the image of God” (eikona theou), which justified a whole Platonic reconstruction of the creation. Not just for Philo, that made it both easy and tempting to assimilate the Biblical stories to Greek philosophical interpretations. Septuagint translations also opened the way to imagining other divine beings. In Psalm 82, God stands in the Assembly of gods, synagoge theon.
Gnosticism thus shares many common intellectual assumptions of its time, but did not emerge inevitably as an offshoot of Platonic thought. What we do see, though, is that between about 100 BC and 200 AD, a number of groups rooted in the Jewish world borrowed extensively from those ideas and long remained in dialogue with Platonic philosophy. That was true of those Christians we think of as orthodox, as well as those we label Gnostics. Both used Platonic thought to frame and develop their ideas, and for much of that intellectual journey, they traveled alongside Middle- and Neo-Platonists
Some major books on these general topics include the collections by Richard T. Wallis and Jay Bregman, eds., Neoplatonism and Gnosticism (State University of New York Press, 1992); John D. Turner and Ruth Majercik, eds., Gnosticism and Later Platonism (Society of Biblical Literature, 2000); John D. Turner, Sethian Gnosticism and the Platonic Tradition (Presses Université Laval, 2001); and Kevin Corrigan et al, eds., Gnosticism, Platonism and the Late Ancient World (Brill, 2013).
Dylan M. Burns’s important book Apocalypse of the Alien God (University of Pennsylvania Press, 2014) explores the relationship between Gnosticism and third century Neoplatonism.