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The term Gnosticism is used by scholars with a wide variety of meanings and levels of specificity. Sometimes the term refers only to those Sethians who used the term gnostikoi to describe themselves. Sometimes it is used more broadly to include Valentinians, followers of Basilides, and others. Likewise, one scholar may consider Simon Magus a gnostic, where another considers him a proto-gnostic. Some early Church fathers, such as Irenaeus, seemed to think that all heresies were Gnosticism at root, and thus that any heretic was in a sense a Gnostic.
Pre-Christian characters important to Gnostics
Gnostics considered many pre-Christian characters to be important religious figures. Adam and his son Seth were especially important. Several figures appear in Gnostic versions of Old Testament stories who do not appear in canonical versions, such as Norea, who saves the Gnostics from the flood in the time of Noah. The three companions of Daniel are called by many names in Gnostic texts, and often invoked. Eugnostos is a proto-Sethian writer of the Nag Hammadi text of the same name, and may have lived as early as the 1st century BC. John the Baptist is sometimes claimed as an early Gnostic leader — for example, by the Mandaeans. Other figures are more difficult to locate in time, such as the Prophets Barcoph and Barkabbas, mentioned by Basilides and Epiphanius.
Likewise, it may not have been unusual for even Christian Gnostics to consider a variety of pre-Christian people as important religious figures. Irenaeus claims that followers of Carpocrates honored images of Pythagoras, Plato, and Aristotle along with images of Jesus Christ. Philo of Alexandria, Zoroaster, and Hermes Trismegistus may have occupied similar roles among other early Christian gnostics.
Christian Gnosticism in the first centuries
Although some scholars hypothesize that gnosticism developed before or contemporaneous with Christianity, no gnostic texts have been discovered that pre-date Christianity. James M. Robinson, a noted proponent of pre-Christian Gnosticism, has admitted "pre-Christian Gnosticism as such is hardly attested in a way to settle the debate once and for all." Since pre-Christian Gnosticism, as such, is strictly hypothetical, any influence of Gnosticism upon Christianity is speculative. Therefore, gnosticism as a unique and recognizable belief system is typically considered to be a second century (or later) development.
Important figures in early gnosticism include Simon Magus (named in the book of Acts; later claimed by gnostics to be a proponent of gnosticism), Cerinthus, Carpocrates, and Basilides. Early teachers such as Marcion, Theudas, and Nicolas of Antioch are more difficult to define as "gnostics" due to variant teachings that do not fit easily within that category.
Jesus is claimed as a gnostic leader by Christian gnostics (hence "Christian gnosticism"), as are several of his apostles, such as Thomas the Apostle, claimed as the founder of the Thomasine form of Gnosticism. Indeed, Mary Magdalene is respected as a Gnostic leader, and is considered superior to the twelve apostles by some gnostic texts, such as the Gospel of Mary. John the Evangelist is claimed as a Gnostic by some Gnostic interpreters, as is even St Paul.
Nicolas of Antioch and Jezebel of Thyatira, mentioned in the Book of Revelation (late first century), are sometimes claimed as leaders of the Nicolaitans. It's unclear just how Gnostic these figures were, but Epiphanius believes that the Archontic Gnostics are descendents of the Nicolatians.
Origins of Gnostic Beliefs
The gnostics interpreted the stories of God and Jesus differently than other Christians. In general, gnostics believed that the created world was bad, and that is was made by a lesser, evil god distinct from the good god. They believed that there is an element of the divine in each person, and that that divinity wants to join again with the good god.They also believed that people could be saved by a sort of secret knowledge. Some thought that it did not matter what people did in life, since the body was not eternal. While Christians thought Jesus was both human and divine, gnostics thought he was divine only, but appeared human. While Christians thought Jesus transcended sin when he died, gnostics say he merely escaped from the physical world. While Christians said people must read the Old Testament, gnostics said people only need to read the New Testament. While orthodox Christians consider them heretics, the Gnostics believe themselves to be Christians. Their views and ideas on Jesus were the result of not having the canonical Bible in the early years of Christianity, giving them plenty of other supposed holy texts that they could read for guidance.
The need for revelation through divine knowledge in order to achieve perfection in a Supreme Deity is important to understand in the identification of what passages there is concerning Gnosticism in the New Testament, which would influence orthodox teaching. Gnostic beliefs that stray from orthodox Christianity include: the creator as a lower being and not a Supreme Deity; the belief that all matter is evil and the body is a prison to escape from scripture having a deep, hidden meaning whose true message could only be understood through “secret wisdom”; and Jesus as a spirit that “seemed” to be human, leading to a rejection of the incarnation.
Gnosticism created a speculative view of the idea of the oneness of God and the “triplicity” of his manifestations. It seems to have taken Neoplatonic metaphysics of substance and hypostases as a departure point for interpreting the relationship of the “Father” to the “Son” in its attempt to define a new theology.
The ancient Nag Hammadi Library revealed how widespread this movement was. The writers of these manuscripts considered themselves ‘Christians’, but owing to their combined beliefs, borrowed heavily from the Greek philosopher Plato. The find included the hotly debated Gospel of Thomas, which parallels some of Jesus’ sayings and early life. This may point to the existence of a hypothesized lost textual source for the Gospels of Luke and Matthew, known as the Q document. 
The 3rd century also sees Bardaisan or Bardansanes, an immediate forerunner of Mani. He was a Valentianian at one point but later rejected them. The prophet Mani, who described himself as "the apostle of Jesus Christ", founded a religion called Manichaeism. His religion borrowed heavily from Gnosticism and may well be thought of a form of gnosticism, so it might be fair to think of Mani as a father of Christian Gnosticism, although clearly many would dispute this.
By the early 4th century, gnostics were kicked out the church and officially forbidden to meet, by the mid 4th century their books were widely banned and by the late 4th century Gnosticism carried a death penalty in the Roman empire. The Sethian Gnostics, Archontic Gnostics, Basilidean Gnostics, Valentinian Gnostics, and Manicheans seem to be the only schools of Christian Gnostics to survive into the 4th century. St. Augustine of Hippo claimed to be a Manichean early in life, but later rejected it, and thus was a Church Father who was at one point a gnostic. Likewise, the late 3rd-early 4th century theologian Lactantius has sometimes been thought of as being influenced enough by Gnosticism to be a Gnostic father, but this is by no means clear.
- "At this stage we have not found any Gnostic texts that clearly antedate the origin of Christianity." J. M. Robinson, "Sethians and Johannine Thought: The Trimorphic Protennoia and the Prologue of the Gospel of John" in The Rediscovery of Gnosticism, vol. 2, Sethian Gnosticism, ed. B. Layton (Leiden: E. J. Brill, 1981), 662.
- J. M. Robinson, "Jesus: From Easter to Valentinus (Or to the Apostles' Creed)," Journal of Biblical Literature, 101 (1982), p.5.
- To this end Paul Trebilco cites the following in his article "Christian Communities In Western Asia Minor Into The Early Second Century: Ignatius And Others As Witnesses Against Bauer" in JETS 49.1: E.M. Yamauchi, “Gnosticism and Early Christianity,” in W. E. Helleman, ed. (1994). Hellenization Revisited: Shaping a Christian Response Within the Greco-Roman World. University Press of America. p. 38. ; Karen L. King (2003). What is Gnosticism?. Cambridge, MA: Belknap Press of Harvard University Press. p. 175.; C. Markschies (2003). Gnosis: An Introduction. London: T&T Clark. pp. 67–69.; cf. H. Koester (1982). Introduction to the New Testament, Vol 2: History and Literature of Early Christianity. Walter de Gruyter. p. 286.; For discussions of “Gnosticism” see Yamauchi, “Gnosticism” 29–61; M. A. Williams (1996). Rethinking "Gnosticism": An Argument for Dismantling a Dubious Category. Princeton University Press.; Gerd Theissen (1999). A Theory of Primitive Christian Religion. London: SCM Press. pp. 231–39..
- Elaine Pagels, The Johannine Gospel in Gnostic Exegesis. Heracleon's Commentary on John. Nashville: SBL Monograph Series 17, 1973
- Elaine Pagels, The Gnostic Paul. Philadelphia 1975.
- Marty, Martin (1987). A Short History of Christianity. Augsburg Fortress Publishers. ISBN 978-0800619442.
- Ehrman, Bart D. Lost Scriptures, Lost Christianity