Irenaeus was a Greek bishop noted for his role in guiding and expanding Christian communities in the southern regions of present-day France and, more widely, for the development of Christian theology by combating heterodox or Gnostic interpretations of Scripture as heresy and defining proto-orthodoxy. Originating from Smyrna, he had seen and heard the preaching of Polycarp, who in turn was said to have heard John the Evangelist, and thus was the last-known living connection with the Apostles.

Christianity is an Abrahamic monotheistic religion based on the life and teachings of Jesus Christ. It is the world's largest and most widespread religion with roughly 2.4 billion followers, comprising around 31.2% of the world population. Its adherents, known as Christians, are estimated to make up a majority of the population in 157 countries and territories. Christians believe that Jesus Christ is the Son of God, whose coming as the Messiah was prophesied in the Hebrew Bible and chronicled in the New Testament.