Plotinus Versus Gnosticism
Probably the best-known book from Plotinus' Enneads is Against Those That Affirm the Creator of the Cosmos and the Cosmos Itself to Be Evil, better known today as Against the Gnostics. It is a book in which Plotinus attempts to distance his philosophy from the philosophy that we now retrospectively call Gnosticism, which he found incompatible with Plato's thought.
A quick disclaimer, Gnosticism is a somewhat loaded term, consisting of a multitude of movements that sometimes have less in common than they do. However, a common thread in Gnosticism is the belief in Gnosis as a way to transcend — or escape — the current plane of existence to a higher plane of existence.
What makes Gnosticism so unique is that Gnostics generally saw our current plane of existence as flawed, created by a creator who, rather than being good, is actually ignorant at best and downright evil at worst. Therefore, they argue, we must try to escape and circumvent the ways in which it holds us captive. This was a point of contention for Plotinus, who viewed the universe as inherently good.
The Problem of Evil
The Gnostics attempted to solve a philosophical problem arising from the religions from which these movements emerged, Judaism and Christianity. That is, if the world was created by an omniscient and inherently good creator, how can evil exist? There have been numerous solutions to this problem, of which this is an early one.
Unfortunately, this is precisely why the fate of Gnosticism was set in stone shortly after its inception. It clashed aggressively with virtually every philosophy — and perhaps more importantly, religion — popular at the time.
Destruction and Revival
Gnostic writings flourished among certain early Christian groups until, perhaps unsurprisingly, they were condemned as heresy by the Church Fathers. Attempts to destroy these texts proved largely successful, resulting in the survival of very few Gnostic texts. At least, until 1945, when a collection of Gnostic manuscripts was found near Nag Hammadi, Egypt.
The discovery and translation of these ancient works coincided with new occult movements in Europe. These in turn were inspired by these new findings and eventually radiated their influence into the general zeitgeist, reviving Gnostic thought among the general public.