The Sethian Gnostics
Perhaps the most important of the Gnostic sects are the Sethians. The term "Sethian Gnostics" is a modern one; and some scholars have even suggested this is a totally artificial classification. But it is apparent that a number of texts do share a great deal in common as regards their cosmology and terminology, so for the sake of convenience we can refer to these as "Sethian".
The Sethians are so-called because, naturally, they hold the biblical character of Seth up as a savior-figure So Seth is to the non-Christian Gnostics what Christ is to the Christians.
According to Professor John D. Turner of the University of Nebraska, Sethianism passed through five phases, which can be listed as follows [John D. Turner, Sethian Gnostcism: A Literary History, p.56]:
(1) During the first century before to the first century of the Commen Era, Sethianism was a non-Christian baptismal sect that considered itself possessing the primordial wisdom revealed to the still human Adam and Seth, and also expecting also a messianic visitation of Seth. This was typical of the Messianic mind-set of the time; Christianity itself began as precisely such a Messianic religion, centred around Jesus. In the case of the Sethians, here we would have the Source material for later writings: Sophia myth, exegesis on Genesis, and the baptisimal rite. I would also include here the Apocalypse of Adam, a very early work that would seem to be transitional between Jewish and Gnostic apocalyptic, in its original form, and the above quoted fragment from The Testimony of Truth.
(2) During the later first to second century C.E. Sethianism became gradually Christianised through contact with Christian baptisimal groups, and identified Seth or Adam with their pre-existent Christ. Here then Seth and Adam are transformed from human to supernatural beings . Representative texts include the Apocalypse of Adam in its completed form, the Apocryphon of John , the Hypostasis of the Archons, Norea, and Trimorphic Protennoia.
(3) During the later second century C.E. Sethianism became increasingly estranged from Christianity, and its own doctrines become more orthodox and codified. Typical here is the Gospel of the Egyptians, perhaps the "classic" Sethian work, and a secondarily Sethianised Jewish piece called Melchezidec (named after the mythical high priest).
(4) By the third century C.E. Sethianism had rejected by the Christian Church, while meanwhile becoming increasingly attracted to the individualistic mystical practices of Platonism, and also adopting Platonic (Neopythagorean, Middle Platonic, and Neoplatonic ) metaphysical and numerological ideas. This is the period of the Church hersiological accounts, and Sethian texts like Allogenes, Zostrianos, and the Three Steles of Seth, which incorporate various Neopythagorean and Neoplatonic ideas.
(5) By the later third century C.E. Sethianism had become estranged from Platonism (Neoplatonism), and was becoming increasingly fragmented into various derivative and sectarian gnostic groups. Texts from this period of decline include Marsenes and the Bruce Codex.
In the later (i.e. 200 C.E. on; stages (4) and (5) above) Sethian writings there is the tendency towards monism , a somewhat more positive attitude toward the material world, and, in Zostrianos and Marsenes, a more elaborate and sophisticated account of the various planes of existence, including the sub-spiritual (sub-pleromatic) realms [Ibid, pp.83-5].