Irenaeus

Irenaeus was a second-century Christian bishop of Lugdunum, now Lyon, whose writings defended apostolic Christianity against Gnostic teachings. His surviving work Against Heresies is a major source for early Christian theology and rival movements.

Gnosticism: A diverse set of ancient religious movements that offered salvation through special knowledge and often distinguished spirit from matter. Irenaeus’s polemic targets teachings grouped under this name, though their movements were diverse.
Against Heresies: A five-book work by Irenaeus that describes and refutes teachings he classifies as heretical. This is his principal surviving argument against Gnostic and related Christian movements.
Polycarp of Smyrna: A second-century Christian bishop of Smyrna and an early martyr, remembered as a disciple of the apostle John. Irenaeus says he heard Polycarp teach, linking his theology to an earlier Christian generation.
Valentinus: A second-century Christian teacher associated with the Gnostic movement later called Valentinianism. His teachings and followers are among the main targets of Irenaeus’s surviving polemic.
Four-Gospel canon: The recognition of Matthew, Mark, Luke, and John as the four authoritative Gospels. Irenaeus explicitly defends these four as the church’s authoritative Gospel collection.
Apostolic succession: The claim that church leaders inherit authority through an unbroken succession from the apostles. Irenaeus invokes public succession to distinguish church teaching from secret claims of authority.
Valentinianism: A Christian Gnostic movement associated with Valentinus, featuring a divine realm of paired emanations and a flawed creator. Irenaeus gives an extensive account and rebuttal of this influential rival teaching.
Smyrna: An ancient Greek city on the Aegean coast, now İzmir in Turkey. Irenaeus’s remembered connection to Polycarp places part of his formation in this city.
Marcion of Sinope: A second-century Christian teacher who rejected the Hebrew scriptures and formed a distinct church. Irenaeus treats Marcion’s scriptural division as a major threat to Christian continuity.
Christian orthodoxy: The body of beliefs regarded as correct within a Christian tradition. Irenaeus’s arguments became an influential early statement of boundaries between accepted teaching and heresy.

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