“Gnosticism” is a term primarily used to describe a variety of related movements in the ancient Hellenistic culture around the Mediterranean. The primary precursors of these movements were the Jewish Apocalyptic (visionary) tradition, ancient mystery religions (ritual and symbolic transformative spiritual practices), and platonic philosophy (note that ancient philosophy was a way of life, not an abstract theory).

The combination of these three major precursors into an early form of Gnosticism (or proto-Gnosticism) can be clearly seen in the teachings of Jesus that appear in the canonical gospels. Contrary to naïve assertions that Greek philosophy, or mystery practices were later alien influences upon the purportedly isolated Jewish origins of Christianity: historically we see a great deal of influence in ancient Palestine from the conquest of Alexander, and continuing under Roman rule. Philo of Alexandria, is a prime example of the Jewish Platonist movement that sent teachers into Palestine, just prior to the time of Jesus. There was also a Jewish Gnostic movement associated with Seth the son of Adam, which exhibits even more obvious platonic influences.

The prophet John the Baptizer was the person Jesus went to for initiation. This is a simple relationship of mystagogue and initiate, that the gospel writers go to great lengths to re-frame so that Jesus has primacy. The followers of John have continued to this day as the Mandeans, a Gnostic group. So these precursors and closely related movements can be traced to the origins of Christianity. The majority of Gnostic movements were Christian and traced their origin through apostles to Christ. However, their emphasis was on the “living Christ,” the continued experienced presence of Christ, rather than on an historical connection.

Gnostics sought personal transformation, this is the metanoia ('turning of mind'; often somewhat mistranslated as “repentance”) that Jesus taught (and John, and philosophers like Plato before him), that would lead to spiritual liberation. The method they used for transformation was referred to as “Gnosis”: an inner or spiritual knowing by means of direct acquaintance or apprehension that is transformative in that it is liberating/redemptive. Sometimes this was written as a Gnosis of Gnosis, or a Gnosis of Gnoses (the plural form). This is not simply the common use of the Greek word gnosis, but a type of metagnosis, which in Greek is related to metanoia. The adjectives “perfect/complete” and “eternal” are also used at times to distinguish this redemptive Gnosis. It is also described as the Gnosis of the divine, or the Gnosis that is possessed by the divine, yet is is also Gnosis of oneself. Gnosis is often linked closely to truth and to freedom, and is the knowing of the truth that sets free. Most often is is described as light, or as the light of the mind, or intellect.

Gnosis was sought within the structure of a symbolic/mythological environment, a loose diagnostic system that could be applied to experiences. The myths and symbols were not literal, nor exact maps: more a topology than a topography. Giving accounts and means of recognizing the various phenomena one experiences in the process of attaining Gnosis. The creation myths, for example, give accounts of the emanation of the divine and into the world, and so provide a sort of guide back to the divine. A similar form can be seen in the sefiroth of the Kabbalah, in that the tree of life structure is both the emanations of God into creation and a sort of map back to God.

In seeking this transformation, Gnostics made use of the techniques of the mystery religions. Using symbolic ritual practices to offer opportunities for personal transformation. Some of these continue to exist, being preserved in the Christian tradition as Baptism, Chrism, and the Eucharist—which are all mentioned and discussed in ancient Gnostic texts. The expressions of the other mystery practices that are mentioned in the texts have been lost. There are also what seem to be books of chants for group practice. Visionary practices were of great importance, and accounts of them were seen as a resource for others. And there is mention of what may be the practice of "putting on the light."

Texts are attributed to specific Gnostic groups based upon unique emphases or aspects. Often these are very recognizable “families” within the larger “clan” of Gnosticism. For example: The figures of Adam and Seth as the primary figures of Gnosis indicate the Sethian movement. Similarly, calling the teaching figure Hermes or thrice-great indicates the Hermetic movement. While, a highly developed symbolic mythology with ritual elements indicate the Valentinian movement.

Which specific groups are included under the umbrella term varies. Historically it is the movements in the ancient Mediterranean that were the targets of the polemical writings of the early heresiologists that were considered Gnostic. More modern classification generally includes groups with texts that were collected in the Nag Hammadi Codices are most often included. Though the Hermetic tradition is sometimes excluded as it was not attacked by early heresiologists.

What other groups are considered to be Gnostic is largely a matter of how it is understood. If it is considered to be a matter of common tendencies and approaches, doctrines, or origins. This in turn is dependent on the purposes of the one making such considerations. Some of the groups that can be seen as part of a larger Gnostic tradition are: Mandeans, Hermeticists, Manicheans, Bogomils, and Cathars.