The Sethians were an ancient Christian sect that worshipped Seth, the third son of Adam and Eve. The group first flourished in the Mediterranean region and may even be older than the first Christian churches.
The sect originated in the 2nd and 3rd century CE and was influenced by both Christianity and Middle Platonism. Platonism deals with the beliefs of Plato where God is seen as a transcendent being who operates through divine intermediaries.
The snake that tempted Adam and Eve in the book of Genesis in the Bible is seen as a heroic figure and not a figure of Satan. The act of Adam and Eve eating the Fruit of Knowledge was considered to be freedom and not adversity. The Sethians believed that Seth was a sort of messiah who could get in touch with God and act as his messenger. Seth was seen as a divine incarnation.
Sethianism was rejected by traditional Christian ideology in the early 3rd century and was further divided into several Gnostic groups which existed until the Middle Ages.
In more recent times in the 1850s, neo-Sethianism became popular in the UK. A group called the Knights of Seth attempted to resurrect the dual ideology of Gnosticism and Christianity. They believed in a true God and a false god. Seth was the messiah who could get in touch with the true God.
The Knights of Seth movement never gained ground and the group gradually became extinct.