Gnosticism derives from the word "gnosis", or knowledge, or act of knowing. Specifically, it is not knowledge obtained by reason (there's another word for that in Greek), but knowledge from personal experience or perception. More specifically, it is secret or hidden knowledge, that available to an elite or to initiates.
The movement known as Gnosticism, though, denotes a prominent, heterodox stream of early Christianity. This stream of Christianity argues that we have access to the absolute truths of human existence. Gnosticism came close to the mainstream of Christianity with Valentinus, who was born in about 100 A.D. But by 180 A.D., church fathers such as Irenaeus published attacks on Gnosticism. It faded, and in fact was mostly lost until 1945, when an Arab peasant near the town of Nag Hammadi in Egypt was digging in his field and came across an earthenware jar. In it were 13 scrolls – the Nag Hammadi library. These scrolls contained the "Gnostic Gospels", the set of writings that Christians had suppressed.
What was in these writings? Several elements are common:
a. Gnostics were not much interested in systematic or dogmatic theology, but rather were interested in knowledge acquired by experience. Gnosis was the "creative experience of revelation, a rushing progression of understanding." Jung: "we find in Gnosticism what was lacking in the centuries that followed: a belief in the efficacy of individual revelation and individual knowledge. This belief was rooted in the proud feeling of man's affinity with the gods..."
b. Harold Bloom gives a second characteristic: it is "a knowing, by and of an uncreated self, or self-within-a-self, and [this] knowledge leads to freedom." Gnostics talked about an "uncreated self", the divine seed, the pearl, the spark of knowing, consciousness, intelligence, light. The seed of the intellect is the same stuff that God is made of, and is man's true reality. So, realizing our true selves means realizing that we and God are one. However, we are not God. This paradox is one of the core truths of Gnosticism, and as a paradox is not accessible to rational reflection.
c. The creator God of orthodox dogma, the one who claimed to have made man, was in fact not God at all but a lying demon. Gnostics call him by many names, none of them very nice – Saklas (the blind one); Samael (god of the blind); the Demiurge (the lesser power). In the Gospel of Thomas, the following is found:
Jesus said, "I am not your master. Because you have drunk, you have become drunk from the bubbling stream which I have measured out"
He who will drink from my mouth will become as I am: I myself shall become he, and the things that are hidden will be revealed to him.
This is, to say the least, not very orthodox by Christian standards.
d. Gnostics had a reverence for texts and scriptures that were not accepted by the orthodox. The Gospel of Thomas is one of these. Some of these texts are very old – Gospel of Thomas is almost certainly as old as any of the canonical gospels. These texts tends to be very mythological, full of stories and allegories.
e. God is conceived as a dyad or a duality. Often this is understood in male/female terms. Elaine Pagels:
One group of gnostic sources claims to have received a secret tradition from Jesus through James and through Mary Magdalene. Members of this group prayed to both the divine Father and Mother: "From Thee, Father and through Thee, Mother, the two immortal names, Parents of the divine being, and though, dweller in heaven, humanity, of the mighty name...
Some Gnostic traditions saw God as a union of disparate natures, or even opposing natures. But the Gnostic tradition continually talks about reuniting the severed components.
Gnostic tradition argues that the world is flawed (many traditions agree on this). Gnostics, though, argue that the world was created flawed, it did not become this way. They recognize that life is suffering. But Gnostics differ from orthodox Christians in that they believe that the source of the suffering is not the initial fall, which was the fault of humans, but rather the fault of the creator. History has been a series of attempts to cover up the flaws, to argue that there is harmony that is at the root of all. However, it has its roots in fact in the flawed starting point, a god that "emanated" the world. The emanations consist of intermediate sub-deities (Aeons). One of the Aeons is Sophia, or wisdom, who emanated from her own flawed being a flawed consciousness, who created the material and psychic worlds. This being was not aware of his own origins, and imagined himself to be god. In fact, he was created by the true god, and was in fact a "half-god". There is the light of the "true God", and the darkness of the "half-god".
Humans mirror the duality found in the world. They are composed of perishable and imperishable parts. Humans are generally unaware of the divine spark within them, and this is aided by the half-god who wants to keep us ignorant of this spark. Humans must rise through the material level to the spiritual level.
But human evolution is not sufficient by itself to bring about spiritual freedom. We need help. This comes from "messengers of the light", some of whom are mentioned in Gnostic scriptures: Seth (third son of Adam), Jesus, the prophet Mani. Most Gnostics look to Jesus as the principal saviour figure (the Soter). Salvation is not from sin, but from ignorance, specifically ignorance of spiritual realities.
The result of salvation is, among other things, ethics. But if this is understood as a system of rules, Gnosticism will have no part of it. Systems are part of the Demiurge's purpose. While Gnosticism is a diffuse set of ideas, sometimes contradictory, in general it encourages non-attachment and non-conformity to the world, being in the world but not of the world, a lack of egotism, a respect for the freedom and dignity of other beings.
Gnosticism does not generally believe that death is a necessary release from the illusion we live in. Some have a belief in reincarnation. The goal for all continues to be striving for the gnosis.