While Gnosticism was a diverse movement, one of the earliest and most influential systems of thought we can identify is Sethianism. Emerging from Hellenistic Judaism, Sethians took familiar stories and inverted them. For them, the true spiritual ancestor of humanity wasn't just Adam, but his third son, Seth. The descendants of Seth were believed to be a chosen, enlightened lineage, carriers of divine sparks trapped in a material world they did not create.

This intricate cosmology is laid out in texts like the Apocryphon of John, or the Secret Book of John. It presents a radical vision of the divine, one that begins not with a creator God, but with a perfect, unknowable entity called the Monad, or the One. From this source, a series of divine emanations, or Aeons, emerged. The very first of these was a feminine principle of thought and foresight named Barbelo.

Together, the Monad, Barbelo, and the other Aeons formed the Pleroma, or the fullness of the divine realm. This was a perfect, immaterial plane of existence. The hierarchy was complex, with pairs of male and female Aeons representing different aspects of the divine mind, such as Intellect, Will, and Life.