Today I'll just give a very brief overview of Gnosticism.
When Gnosticism flourished among Christians in the mid-second century, it had already had a long history. Its presence was more or less underneath the surface, though. Up to that time, it doesn't appear to be a religion in and of itself, rather a religious atmosphere home to a set of ideas that could be joined to any religion.
Gnosticism basically assumed that a person felt trapped in a world that oppresses him, a world in which he or she feels like a stranger. Considering the extension of evil in the world–as did the Marcionites–, or the inanity of matter itself, many people are long for freedom from this world and to join themselves in some way to the divinity in which they believe. It's like two opposites that resist each other, when they should really be joined together.
In this way, Gnosticism was relevant to different spiritual systems, either as a foundation or touching on some of their most basic tenets. In the eastern Mediterranean world, Gnosticism could manifest itself as its own religious atmosphere, thus its own religion. There was a sort of "ground floor" of a religion that, for example, didn't really stress that tear away from the world. But then there was a higher level where true "connoisseurs" of Gnosticism, who desired to possess all knowledge about everything, could receive the special revelation that they were longing for. You can see where this is going. These Gnostics were the elite, lifted high above everyone else. Supposedly, they were worthy of receiving this extra revelation that answered the core questions of a person who pondered religious things, such as:
Who am I?
Where do I come from?
Do I have a relationship with God?
How will I get back to the place from where I came? I.e., how can I reach "salvation?"
How can I eliminate all of the impediments that are trying to keep me from reaching this salvation?Not only was Gnosticism provide a way for people to get answers to these questions, research has considered Gnosticism the legacy of esoteric reflection and ways of understanding–coming both from the Mediterranean and beyond (e.g., Egypt, Mesopotamia, and even India)–the depths of the divine, the mysteries about the creation of the universe, and God's relationship with man. In Greece, and the world it influenced–the eastern Mediterranean, the cradle of Christianity–there have been traditions of secret religious knowledge all the way back since the Orphic times (7th and 6th centuries B.C.), the Pythagorean philosophers and Plato. All of these lead in some way to Gnosticism.