Last Updated September 5, 2023.
St. Irenaeus, bishop of Lugdunum (i.e. modern Lyon, France) wrote his treatise Against Heresies (Latin: Adversus Haeresis, Greek: Ἔλεγχος καὶ ἀνατροπὴ τῆς ψευδωνύμου γνώσεως, translated as "On the Discovery and Overthrow of So-Called Gnosis") as a refutation of particular types of heresies, all of which fell under the general category of what is commonly called "Gnosticism." Gnosticism is a variety of early Christianity that represented a general splintering from what was slowly developing (through writings like Irenaeus, et al.) as orthodox Catholic doctrine.
In broad strokes, Gnosticism is the contention that there existed from the apostolic age a secret doctrine, passed from Jesus to the apostles orally and available only to those initiated into this select group. There were various proponents of Gnosticism in the ancient world (and in fact Irenaeus is one of the main sources of these individuals' teachings). They all had slightly different doctrines, but they all generally espoused the idea of salvation through secret knowledge, not faith.
The primary teacher of Gnosticism (and so character in Irenaeus' treatise) is Valentinus. He has an elaborate cosmology (discussed in book I of Against Heresies) which states that the creator (or demiurge) is imperfect and is different from the supreme being or God from the Holy Scriptures.
Other Gnostics discussed by Irenaeus include Basilidies (of Alexandria), who claimed that human beings were reconstituted into new bodies after death (a process known as metempsychosis, which translates to something like "transmigration of the soul"). Like Valentinus, Basilides has an elaborate cosmology involving Nous, Logos, and Dunamis (personifications of the Greek works for Mind, Reason, and Power, respectively).
Two additional figures in the tradition of Gnosticism whom Irenaeus discusses are Cerdo and his pupil Marcion of Pontus (in modern Turkey). These two rejected the Old Testament, teaching instead that the God of the Old Testament was a tyrannical demiurge, and that the (different) God of the New Testament is benevolent.