Irenaeus (; Greek: ) (early 2nd century – c. AD 202), also referred to as Saint Irenaeus, was Bishop of Lugdunum in Gaul, then a part of the Roman Empire (now Lyon, France). He was an early Church Father and apologist, and his writings were formative in the early development of Christian theology. He was a hearer of Polycarp, who in turn was traditionally a disciple of John the Evangelist.