Until the 20th century the works of Irenaeus and other heresiologists (orthodox Christian writers who described unorthodox groups) were the principal sources of information about gnostic movements. Only a handful of manuscripts containing the authentic writings of such groups were known; they existed primarily in two sets of Coptic texts, the Askew Codex and the Bruce Codex, which were discovered in Egypt in the 18th century but not published until the 19th century. A third important Coptic text, known as the Berlin Codex 8502, was announced in 1896 but not published until the mid-20th century. In 1945, 12 additional codices and parts of a 13th codex, all probably dating from the 4th century, were discovered near the town of Nag Hammadi (now Najʿ Ḥammādī) in Egypt. The Nag Hammadi collection contains Coptic translations of more than four dozen writings that are diverse in type and content, including “secret sayings” of Jesus, non-Christian works belonging to the Egyptian Hermetic tradition, theological treatises, and lengthy mythological stories. Many of the works also contain doctrines or myths that were condemned by Irenaeus and other heresiologists.

Among the Nag Hammadi writings are three separate copies of the Apocryphon of John, an especially important gnostic myth; a fourth copy is included in the Berlin Codex 8502. Corresponding closely to the myth that Irenaeus ascribed to the sect called gnostikê, the Apocryphon purports to be a secret revelation from Jesus that was received in a vision by the apostle John. It conveys the true nature of the divine realm and its relationship to the material cosmos and humanity. While the transcendent god or invisible spirit is inconceivable and ineffable, the pleroma (Greek: “full perfection”) of the divine is a hierarchical family of personified aeons, who emerge as the fruit of the spirit’s self-contemplation or self-expression. For example, as in the myth described by Irenaeus, Barbelo emerges as the first thought of the transcendent god, and she is soon accompanied by Foreknowledge, Incorruptibility, Eternal Life, and others. The imperfect material realm is understood as a copy of the perfect spiritual realm, an idea partly derived from the Platonic doctrine of ideas, or forms. The myth also draws on the biblical theme of humanity as formed in the image of God (Genesis 1:26–27); true or divine humanity, however, is that spiritual family brought into being in the realm of perfection as the spirit’s image. That realm is the dwelling place of the spiritual Adamas, his son Seth, and the race or offspring of Seth.

The creator of the visible realm and of the earthly Adam and Eve of the biblical Garden of Eden is a lesser being, a ruler (archon) named Ialdabaoth, who is a dark caricature of the creator God of Genesis and the demiurge of Platonism. Wisdom, the lowest entity in the realm of perfection, creates Ialdabaoth in an unauthorized attempt to produce a likeness of herself. Ialdabaoth in turn creates the material cosmos and rules it with subordinate powers who are his own imperfect offspring. A willful and malevolent figure, Ialdabaoth is unaware of any power above him and is easily duped by providence into actions that either serve divine ends or are stymied by countermeasures from the divine realm. He does not realize that his cosmos is patterned after a more transcendent realm, and he ignorantly boasts that there is no god above him.

When, in response to that declaration, the image of the divine humanity above is revealed on the waters below—an allusion to Genesis 1:2 (“the earth was a formless void and darkness covered the face of the deep, while a wind from God swept over the face of the waters”)—Ialdabaoth and his rulers fashion an Adam in its likeness. Wisdom then tricks Ialdabaoth into breathing life into the figure, an act that empties him of what power he had received from Wisdom and transfers it to Adam. The spiritual power now within Adam is portrayed as a feminine entity who supplies him with insight that makes him disobedient to Ialdabaoth. The latter then attempts to deprive Adam of his power by putting him to sleep, extracting the power from Adam’s rib, and molding it into the shape of a woman. But Ialdabaoth’s plan fails, for, in this revision of the biblical myth (Genesis 2:21–23), when Adam awakens and beholds the woman, Eve, he experiences an even deeper insight, an awakening from the “drunkenness of darkness.” Enraged, Ialdabaoth casts the couple from paradise, introduces sexual desire, and seduces Eve and begets from her Cain and Abel. Because their father is an oppressive archon rather than a human, however, Cain and Abel are the same. As archons, they rule over the material elements (fire, wind, earth, and water) and therefore also over the material bodies of future human beings. However, Adam begets his son Seth in the likeness of the divine Seth, son of Adamas, the prototype of ideal humanity. The human race is thus spiritually the seed of Seth, though bodily incarnation at birth entails a forgetting of that divine origin. The realization of one’s spiritual ancestry must be reawakened by revelation.

The theme is from Platonic philosophy, illustrated in the myth of Er in Plato’s Republic, in which a slain warrior named Er is revived briefly on his funeral pyre and tells of what he has seen of the fate of souls after death. The lengthy account includes a description of reincarnation and of the necessity of each soul to drink of the river of Forgetfulness before coming into another body. According to the Apocryphon, until a soul is saved by receiving revelation of its true identity, it continues to experience further reincarnations. If souls knowingly reject the revelation, they will suffer eternal damnation.

Several Nag Hammadi texts include myths that are similar to those of the Apocryphon of John. The tradition has sometimes been labeled “Sethian” because of the prominent role of the figure of Seth in several of those works. The origins of the Sethian mythology remain uncertain, but it may have emerged prior to the birth of Christianity or apart from Christianity in heterodox Jewish circles. It could then have been adapted by Christian writers who identified Jesus with the myth’s original revealer figure. In any event, there is significant diversity among the so-called Sethian sources, and they are probably best viewed as products of different stages of a complex series of religious innovations.