Gnosticism (after gnôsis, the Greek word for “knowledge” or “insight”) is the name given to a loosely organized religious and philosophical movement that flourished in the first and second centuries CE. The exact origin(s) of this school of thought cannot be traced, although it is possible to locate influences or sources as far back as the second and first centuries BCE, such as Zoroastrian Scriptures, Vedic philosophy, the early treatises of the Corpus Hermeticum, the Jewish Apocalyptic writings, and especially Platonic philosophy and the Hebrew Scriptures themselves. Gnosticism is the teaching based on Gnosis, the knowledge of transcendence arrived at by way of interior, intuitive means. Although Gnosticism thus rests on personal religious experience, it is a mistake to assume all such experience results in Gnostic recognitions. It is nearer the truth to say that Gnosticism expresses a specific religious experience, an experience that does not lend itself to the language of theology or philosophy, but which is instead closely affinitized to, and expresses itself through, the medium of myth. Indeed, one finds that most Gnostic scriptures take the forms of myths. The term “myth” should not here be taken to mean “stories that are not true”, but rather, that the truths embodied in these myths are of a different order from the dogmas of theology or the statements of philosophy.
The Gnostic world view is sometimes characterized as anti-cosmic, but in reality it is merely critical of the cosmos, because of its numerous flaws. Like Buddhists, Gnostics came to the recognition that earthly life is filled with suffering. Some of this suffering is undoubtedly of our own doing, but certainly most of it originates in the natural or cosmic order itself. Why do virtually all creatures sustain themselves by eating each other? Why are living beings destroyed by natural catastrophes? Why do humans, in addition to all other difficulties, also suffer depression, alienation and boredom? Because the causes of these and other conditions are inherent in the fabric of the world so said the Gnostics. Contemporary scholarship holds that what we call “Gnosticism” was a diverse movement, showing many complex characteristics. Yet, it is quite evident that this wealth of diversity in myth, teaching and perhaps in practice possesses an undeniable central core. While there may have been numerous Gnostic teachers and schools, this does not mitigate against the fact that there was one Gnosticism; what united the various Gnostic orientations was more important than what divided them. It is also important to keep in mind that one of the uniting factors was a common dedication to the founder of the Christian tradition, Jesus Christ. For the sake of clarity it is useful to confine the term “Gnostic” to the kind of person we see in the Nag Hammadi writings; a Christian, albeit of a singularly creative and heterodox kind, especially when compared with Christians of so-called mainstream orthodoxy. It is true that the term “Gnosis” and much of the basic Gnostic view of reality were shared by people who were not Christians, but these “pagan Gnostics” should properly be called Hermeticists, for they employed the figure of the Greco-Egyptian Hermes as their savior, very much like the Christian Gnostics were to do with Jesus. It is also legitimate to speak of something that Gershom Scholem has tentatively described as “Jewish Gnosticism” but the accurate name for its earlier development may be Essene and Merkabah mysticism and for its later manifestation one may properly use the term “Kabbalah”. 
“But it is perhaps desirable to state unequivocally that the teachings here, however, fragmentary and incomplete belong neither to the Hindu’s, the Zoroastrian, the Chaldean, nor the Egyptian Religion, nor to Buddhism, Islam, Judaism or Christianity exclusively. The Secret doctrine is the essence of all these.” _The Secret Doctrine, Madame Blavaskty.
While it has many writings from its proponents, it has no written standard setting forth its beliefs. For that matter, it has no oral standard, either. (In other words, it is not a determined, well defined teaching that is simply trying to be passed down orally from generation to generation.) Gnosticism is represented in a large library of writings, which was greatly augmented by the discovery of the Nag Hammadi (a city in upper Egypt) “library” in 1945 (about a dozen scrolls in glass jars which were dug up in a field). Their writings include various “gospels” (“Of the Twelve,” “Of Peter,” “Of Philip,” “Of Matthias,” “Of Thomas,” “Of Mary,” “According to the Egyptians”) and several books of “Acts” (“Of Peter,” “Of Andrew,” “Of John,” “Of Thomas”) as well as several oriental writings (including one large collection) of gnostic writings unrelated to Christianity.
Because Gnosticism is subjective and has no written standard, it is also flexible, adapting its approach to the prevailing culture. Regarding guidance given to help the follower know how to conduct himself/herself, we find this:
“The present period of Western culture perhaps resembles in more ways that of second and third century Alexandria. It seems therefore appropriate that Gnostics in our age adopt the attitudes of classical Alexandrian Gnosticism, wherein matters of conduct were largely left to the insight of the individual.”
Gnosticism is not the only religious system that has been taught in many schools of thoughts down through the centuries. Other religions, though, typically have a written standard from the various schools start, so a new-comer to the religion can at least choose a school of thought which seems most consistent with his or her understanding of the standard. Remember, though, that Gnosticism is by definition intuitive and subjective, so we again come up against the problem that there is no basis for claiming that one way of thinking is better than another.