The origins of Gnosticism are normally discussed in terms of debates within Christianity. However, one richly informative conflict occurred beyond the familiar realm of church history.
One of the great minds of Late Antiquity was the Egyptian-born philosopher Plotinus, the leading figure of Neoplatonism, and a younger contemporary of Origen. Around the year 263, in Rome, Plotinus engaged in a furious debate with some Gnostic thinkers. Although the two sides shared many assumptions and terminology, Plotinus condemned his enemies for what he saw as their gross misunderstanding of Platonic philosophy. Among other complaints, he warned that their radical elitism would lead them into misconduct and immorality. Effectively, he expelled these Gnostics from the mainstream philosophical world of the time, after a long period in which Platonists and Gnostics had coexisted and debated together.
That story is quite well known, but recent work has shed major light on just who these Gnostics were. I am referring to Dylan M. Burns’s excellent recent book Apocalypse of the Alien God: Platonism and the Exile of Sethian Gnosticism (University of Pennsylvania Press, 2014). This ambitious and wide-ranging work identifies Plotinus’s Gnostic foes as Sethians.