Gnosticism is some thing I find fascinating and it's something that is very seductive. Here is the point where I would try to define gnosticism, unfortunately defining gnosticism is like trying to nail jello to a wall or prove whether or not Barack Obama is a natural born citizen. It's a subjective ,every flowing ,ever changing thing.
Their are a few points I think I can corral in general.
First a twisting of the Bible. Like in my last blog, many gnostics consider God as portrayed in the old testament as a false, dictatorial, entity that's not actually God and they see Satan as portrayed in the garden in Eden as a liberator or savior who brought man out of ignorance.
Second they don't see Jesus as god or savior of man. In gnostic thought line that I am most familiar with, Jesus is not even seen as actually existing. He is seen as a savior archetype and his mythological death and resurrection was not to save men from their sin but to reconcile the fallen "Sophia" to the unknowable god.
Third that God did not create the universe but he did. Ok this is where gnosticism gets a little confusing. The Unknowable god Created the Heavenly realms and after that their was just a bunch of junk (matter) laying around. At some point comes along this "Demi-urge" character who thinks he's God and from the junk laying around fashions the material universe and calls himself god.
Next is the character of Sophia. Sophia is a Greek word that means wisdom. In gnostic mythology Sophia or wisdom has been personified as a goddess with her own storyline and various incarnations. In fact she's actually seen as the savior of man and not Jesus Christ. When in gnosticism like with Kabbalah when talking about the various heavenly or diving entities they are spoken of as emanations from the one "unknowable" god. Sophia was one of the last emanations from the one and wanted to create something without paring with the one and actually created a deformed blind arrogant creature who was the Demi-urge. (it seem Wisdom wasn't so wise) and thus causing her to be separated from "the one".
Many gnostics also have espoused the idea of reincarnation as well as other eastern ideas. Many a hatred of mainstream Christianity especially Catholicism, anything with orthodox in the name, and any that consider themselves fundamentalist and of this created world in general seeing matter as evil. As they see them as worshiping the evil demi-urge. Another strong trait is that they don't recognize the authority of any scripture really, they take ideas from the Bible, nag hammadi, and other apocryphal and psuedopigraphical works though seeing non of them as the ultimate authority. And still others look to other works such as the writings of the mystery schools, works of Jung, works from Philip K. Dick, and even movies such as "The Matrix" and "Dark City" for spiritual ideas.
Gnostics often claim to be the christianity before christianity and were spoke about in the Bible as false teachers. Some claim Paul as a gnostic saint as he never met Christ in the flesh but had a vision of him on the way to Damascus. Though I have listed some common points here the stated goal of gnosticism is supposed personal revelation of god or "gnosis". Which is fine, Paul talked about having spiritual knowlege of God, he also talked about testing it under the light of scripture and if it didn't measure up it should be considered the teaching of demons (tim 4:1) . The gnostics take all spiritual wisdom as good whether and especially if doesn't agree with scripture.
This is just a very very quick over view of gnosticism, as I said gnosticism is like looking an ever shifting desert almost impossible to pin down, Knowledge from God is not though, James the brother of Jesus Christ writes, Every good thing bestowed and every perfect gift is from above, coming down from the Father of lights, with whom there is no variation, or shifting shadow. (jam 1:17), and like I said above, while Gnosticism is extremely seductive and it tends to speak a language similar to Christianity it is a shifting shadow.