Gnosticism refers to a group of associated beliefs and movements, including some grounded in Judaism, some in Christianity, and some in Neo-Platonism. We have two main sources for our study of gnosticism, the reports of their opponents, especially Irenaeus, a Christian bishop who wrote against them because he considered them heretics, and a small group of writings by various Gnostics recovered by archaeologists from the desert in the 20th century. These writings have in common a dualistic cosmology, assuming a benevolent God and an evil figure, usually the Demiurge, who created the world. Matter and the world as we know it are corrupt, but it is possible to have some direct gnosis or intuitive knowledge of the true God. The Aeons are emanations of God and the pleroma the totality of the divine.