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.
According to gnosticism, all of nature is a mistake produced by an ignorant god.
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.
Modern authors who believe (or, more likely, wish) gnosticism was the true, original Christianity—an idea with no historical support—love to portray early Christian apologists as hateful, jealous defamers of true gnostic beliefs.
Time, however, has proven men like Irenaeus and Tertullian to be quite honest and accurate in their dealings with gnosticism.
The descriptions I'm giving you of gnosticism come from my familiarity with Irenaeus' Against Heresies, a large work against gnosticism written around A.D. 185. As you'll see from the quotes on this page, the texts that have been discovered in modern times have simply verified that what Irenaeus said was true.
The Gospel of Judas (that's a link to National Geographic's translation of the gospel) says, for example:
There is a great invisible Spirit, which no eye of angel has ever seen, no thought of heart has ever comprehended, and it was never called by any name.
… He said, "Let an angel come into being as my attendant."
A great angel, the enlightened divine Self-Generated, emerged from the cloud. Because of him, four other angels came into being from another cloud, and they became attendants for the angelic Self-Generated.
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:
He said, "Let an enlightened æon come into being," and he came into being. He created the second luminary to reign over him, together with myriads of angels without number [note: for the second time], to offer service. That is how he created the rest of the enlightened æons. He made them reign over them, and he created for them myriads of angels without number, to assist them.
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:
Let us now look at the inconsistent opinions of these heretics—for there are some two or three of them … they do not agree when discussing the same points. (Against Heresies I:11:1)
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 Apocryphon of John describes the creation of the Demiurge by Sophia in these words:
Something imperfect came out of her, different in appearance from her. … She gave rise to a misshapen being unlike herself. Sophia saw what her desire produced. It changed into the form of a dragon with a lion's head and [its] eyes flashed thunderbolts.
In The Apocryphon of John, Sophia did at least know she produced this Demiurge, but she hid him:
Sophia surrounded him with a brilliant cloud … so that no one would see it. She named him Yaldabaoth.
But just in case you were going to think that Yaldabaoth might be a good guy, the writer adds, "Yaldabaoth united with the thoughtlessness within him. He begot ruling authorities."
The rest of the story is in the text on this page.
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:
Yaldabaoth modeled his creation on the pattern of the original realms above him so that it might be just like the indestructible realms. Not that he had ever seen the indestructible ones. Rather, the power in him, deriving from his mother, made him aware of the pattern of the cosmos above.
When he gazed upon his creation surrounding him, he said to his host of demons, the ones that had come forth out of him: "I am a jealous God and there is no God but me!"
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.
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.
Much of the gnostic writings I've been looking at seek to be mystical and profound, but really end up senseless. Here's a gem from The Gospel of Thomas:
When you make the two one, and you make the inside like the outside and the outside like the inside, and the above like the below, and when you make the male and female one and the same, so that the male not be male nor the female female; and when you fashion eyes in the place of an eye, and a hand in the place of a hand, and a foot in the place of a foot, and a likeness in the place of a likeness; then you will enter [the kingdom of heaven]. (Saying 22)
Deep? Or just nonsense?
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.