This chapter describes the 'Gnostic school of thought' and the diverse strategies of self-differentiation that it used and elicited. Irenaeus reports that 'Valentinus adapted the fundamental principles of the so-called Gnostic school of thought to his own kind of system' and gives a summary of a myth that the Gnostics taught. The Gnostic myth was a bold attempt to explain the origin and fate of the universe through a combination of the Jewish scriptures and Platonist mythological speculation. Marcion, Valentinus and Justin Martyr developed a set of responses to the Gnostic sect and/or each other that their successors borrowed and developed. These strategies ranged from outright rejection through heresiological rhetoric and withdrawal of fellowship, to adaptation and Christianisation of the Gnostic myth, to more philosophical modes of authority. If the construction of a 'Gnosticism' obscured the characters of the persons and groups assigned to it, likewise the category 'proto-orthodox' can homogenise and so distort the diversity of pre-Constantinian Christianity.

Christian contact with the Sethian Gnostics must have occurred rather early, for by 125 CE one finds Basilides of Alexandria expounding a sophisticated and completely Christian Gnostic theological system. His younger contemporary, Valentinus, likewise developed a wholly Christian Gnostic theology, which reached a high level of sophistication in the work of his pupil Ptolemaeus. One of the first things to strike a reader of Gnostic literature is the vast number of metaphysical entities. One such is the Apocryphon of John that is an early example of what may be called classic Sethian Gnosticism. The Christian philosopher and earliest commentator on early Christian writings Basilides of Alexandria was, in the words of Hegel, 'one of the most distinguished Gnostics'. Ptolemy was described by St Irenaeus as 'the blossom of Valentinus' school'. The last mention of late-antique Gnosticism is to be found in a seventh-century Christian canon prohibiting certain sects, of which that of the Valentinians is mentioned by name.