The word gnōsis is Greek for knowledge. In Gnosticism it does not mean information or belief. It means direct recognition of where you come from, what you are, and how you can return. Between the first and third centuries CE, many different Gnostic schools flourished in the Roman world, among them the Valentinians, the Sethians and the followers of Basilides. They disagreed on many details but shared one core conviction: the material world is not the highest reality.
Most of what we knew about them came from their opponents. Church writers such as Irenaeus wrote long attacks on Gnostic teachings, and the Neoplatonist Plotinus criticised them in his Enneads. The picture changed in 1945, when a jar with thirteen leather-bound codices was found near Nag Hammadi in Upper Egypt. Among the texts were the Gospel of Thomas, the Gospel of Truth and the Apocryphon of John, now read in the Gnostics' own words.