A religious movement that began around the same time as Christianity, which tried to hijack Christianity in the first and second centuries.
Gnosticism believed that the world of matter was evil, that the God who created the world of matter was similarly evil, that there were several Aeons between the true God and the world of matter, who became more and more evil the closer they got to the world of matter.
The number of Aeons believed to exist all depended upon which sect of Gnosticism one belonged to.
The main link between these sects was that Divine knowledge (gnosis) could free man from the effects of the evil of matter and make him a more spiritual being.
Christ, in the more "Christian" of these sects, was seen as being sent to the world to free us from matter by giving us the saving gnosis. Christ in this belief was considered to be a being of pure spirit who took on the appearance of flesh.
As previously noted, more information can be found in St Irenaeus, the more English title of his work being Against Heresies.