Gnosticism, which gets its name from the Greek word gnosis ("knowledge") was a religious movement beginning, possibly, before the time of Christ and extending into the first few centuries of the Christian era. Gnostics viewed themselves as "those who know." Their heretical teachings varied from group to group and can't be pinned down with specificity, but common gnostic beliefs included these:
1. Although Christ appeared to be human, his humanity was merely an illusion.
2. Christ appeared to die, but did not really die. The Crucifixion was really a crucifiction.
3. Christ was not truly God, the second Person of the Trinity. He was merely a created being who was the lowest of the aeons, a group of semi-divine beings between God and man. Each lower aeon was given power by a higher aeon. Christ, the aeon furthest removed from God, created the world because God was too pure to dirty himself with matter.
4. Matter is evil, so one can do anything one wants with one's body, including killing it to release the soul from its imprisonment.
5. The God of the Old Testament is evil, as evidenced by the fact that he created the material universe. He is not the same as the God of the New Testament, who is the God of Love, as Jesus and his apostles taught (1 Jn 4:8, 16).
6. People are saved by acquiring secret knowledge (gnosis), which is imparted only to the initiated.
Gnosticism was similar in some ways to the modern New Age movement. Like New Agers, gnostics used Christian terminology and symbols, but placed them in an alien religious context that gutted the essential teachings of Christ. It's unclear when gnosticism began. Many Church Fathers thought gnosticism was founded by Simon Magus, the Samaritan sorcerer who converted to Christianity (Acts 8:9-24). Some contemporary scholars think gnosticism started a few centuries before Christianity and then invaded it from the outside through the conversion to Christianity of Jewish and Gentile gnostics. Other scholars believe gnosticism started as a Christian heresy.
It seems clear, though, that the apostles themselves had to contend with a form of gnosticism (Col 2:8, 18; 1 Jn 4:1-3; Rv 2:6, 15). Paul said, "Avoid profane babbling and the absurdities of so-called knowledge [gnosis]. By professing it some people have deviated from the faith" (1 Tm 6:20-21).