Irenaeus of Lyon was a Greek-speaking Christian bishop in the 2nd century CE. He lived in Lugdunum, modern-day Lyon, in Roman Gaul. He was born in the eastern Roman Empire, likely in Smyrna, where he was a student of Polycarp, a figure connected to the earliest generation of Christian apostles. This link to the apostolic tradition was central to his life and work. He became bishop after his predecessor was martyred in 177 CE. Irenaeus worked to maintain unity among early churches, and the exact date of his death is unknown, though tradition places it around 202 CE.

His writings were dedicated to defending core Christian teachings against rival movements, particularly Gnosticism. His major surviving work is the five-volume treatise Against Heresies, which systematically argues against Gnostic beliefs. Another work, the Proof of the Apostolic Preaching, survives completely in an Armenian translation. Several other writings are known only through fragments or references by later historians.

Irenaeus is considered a foundational figure in the development of Christian theology and institutional identity. According to modern scholars, he is often called the first great Catholic theologian. His work provided the most detailed contemporary refutation of Gnostic thought, preserving knowledge of those systems. Theologically, he made significant contributions to defining the canon of Scripture, the doctrine of apostolic succession, and the concept of Christian unity based on publicly taught tradition. His arguments helped establish the criteria for orthodoxy that shaped the early Christian church.