Gnostic beliefs have varied over the centuries. I will focus mainly on the 2nd century, the earliest gnostics we know about.
The defining characteristic of gnosticism is "dualism." Dualism means that there's a spirit world, which is good, and a material world, and everything about that material world is bad.
The reason that the material world is bad, according to gnostic beliefs, is that the true God of all the universe didn't make it. An ignorant but powerful creature, accidentally produced by an "æon" made it. We'll explain æons in a moment.
According to gnosticism, the true God is unknowable. Early gnostics often called him Bythus, the Greek word for depth or profundity.
This Bythus then emanated or produced in some unknowable way beings called "æons." Irenaeus, a Christian writing against the gnostics (the Valentinans in particular), mockingly described it as " … produced, but did not simply produce, so as to be apart from themselves." Gnostics loved this kind of esoteric language.
Often Bythus would have a consort. The first æons would be produced from their union. Then, those æons—basically emanations of God—would produce more æons, generally in groups of four, eight, or twelve. These were all mystical numbers to the gnostics, who loved mysticism. It's almost impossible to find anything practical about how to live in any existing gnostic writing.
The Gospel of Judas (that's a link to National Geographic's translation of the gospel) says, for example:
At this point the text of the Gospel of Judas is damaged. However, the passage just quoted is describing the unknowable God and his attendant, who produce other beings.
The text has the angel saying a couple times, "Come into being," but what or whom he's creating is gone from the papyrus (which is 1700 to 1800 years old!!). Then he creates "the first luminary" and myriads of angels. Then we're introduced to the æons:
In most gnostic systems, there were 30 æons. That number was mystical to them, too, being the number of days in a month. These æons dwelt in a place called the pleroma, the Greek word for fullness.
That's the spiritual realm. Spiritual realms will always be described in strange terms, no matter what religion you belong to. However, it's on the earthly realm that gnostic beliefs really get far out …
Keep in mind that there were many gnostic belief systems. They shared some things in common, but because gnostic teachers could form their own schools and obtain fame from some new insight, there were always new "insights" coming from them.
Irenaeus tells us, for example:
Basically, however, the creation of the world by gnostic beliefs goes like this:
Sophia, one of the æons, whose name means wisdom, deeply wanted to know Bythus, the unknowable Father. Because she could not find him or know him, she left the pleroma and the other 29 æons to look for him.
When she could not find him she mourned that she was all alone and that she could not know God.
Unbeknownst to her, her travails and sorrows produced a being, called the "Demiurge," who would become the God of this world and of the Israelites. "Demiurge" is derived from the Greek word for artisan and refers to the fact that the earth is his handiwork.
Sophia finally returned to the pleroma, completely unaware that the Demiurge had been produced.
The Demiurge was left alone. He was as ignorant of Sophia as Sophia was of him. He knew nothing of the great God Bythus, nor anything of the pleroma.
He then decided that he must be God and created the world, something that by gnostic beliefs should never have happened.
From there, according to gnostic beliefs, everything taught in the Hebrew Scriptures, our Old Testament, happened. Yahweh, however—the God of Israel—was really the confused Demiurge. He thought he was the one who created all things and was from the beginning, but he wasn't. As a result, his laws were useless and not beneficial.
One gnostic writing we still have is called The Apocryphon of John (text is here). It gives the Demiurge the name of Yaldabaoth. Here's its description of his creation:
These are the gods of the gnostics.
"Gnostic" comes from the Greek word gnosis, meaning knowledge. It was around early, for Paul refers to "knowledge [Gr. gnosis], falsely so called" (1 Tim. 6:20).
Gnosticism was known as gnosticism because salvation was achieved through knowledge. As an adherent of a gnostic school, you would learn its teachings about the spiritual world and learn to reject the physical world of the Demiurge.
You would especially learn to reject the God of the Jews, whom they equate with the Demiurge, and from whom they got most of their references to him.
This knowledge would allow you to partake of the spiritual realm and its power while on earth, and that same knowledge would allow you to enter the pleroma after you die.
How this happened really varied. Some gnostic groups were entirely wicked, teaching that the body doesn't matter, so sin doesn't matter, either. John addresses gnostics like these in 1 John, telling his readers that the evidence of knowing God is a righteous life and obeying the commandments of God. Other groups took an opposite tack, completely denying the desires of the body with an ascetic lifestyle.
The Apocryphon of John is one of the rare gnostic writings that says anything practical—or even decipherable— about salvation in the gnostic belief system, so let's look at it.
The Apocryphon of John teaches a way of salvation remarkably similar to Christianity. It says that to be saved, a soul must receive power from "the Spirit of Life." This Spirit will deliver them from the false power of the "artificial spirit" of Yaldabaoth.
Once a person receives this Spirit of life, then they "expunge evil from themselves" and "achieve freedom from rage, envy, jealousy, desire, or craving."
Those that do not achieve salvation in this life are sent back again (reincarnation) until they do gain knowledge, receive the Spirit of life, and attain to salvation in the pleroma. Then they will no longer be reincarnated.
Gaining knowledge and turning away from it will result in eternal punishment. No more chances.
As I said, though, salvation varied from gnostic to gnostic. There were groups that taught that some people are purely spiritual, having received something from the pleroma when they were born. Those people—usually the gnostic teachers themselves—could live any way they pleased. They were automatically saved.
Others were part spirit and part soul, and they were required to live righteous lives to be saved. This was usually a reference to those in the Church.
That belief was ingenious because it justified the fact that these gnostic teachers did not descend from the apostles. As you might imagine, it could be hard to claim to be teaching the message of Jesus when your message conflicted with that of Jesus' only companions. However, if the Church consisted of people who needed a different message than typical gnostic beliefs, then you could say that Jesus kept the apostles around for that purpose. In the meantime, he passed his true knowledge on in secret to Mary Magdalene, Lazarus, or some other person.
Those who held these sort of gnostic beliefs also said there were people who were purely soul and no spirit, and such cannot be saved.
This, then, is a basic summation of gnostic beliefs. It is impossible to narrow them down to just one system, but this system was the most typical in the 2nd century.
This picture of gnostic beliefs emerges from the writings of those who argued against them combined with gnostic writings we've found over the last century or so. The Christians who wrote against gnosticism have been much-maligned for portraying the gnostics falsely, but as you can see from the quotes I've given you, their description of gnostic beliefs was quite accurate.