Irenaeus was a Greek Bishop from Polycarp's hometown of Smyrna in Asia Minor, born during the first half of the 2nd century. He was 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 heresy and defining orthodoxy.

During the persecution of Christians by Marcus Aurelius, the Roman emperor, from 161 to 180, Irenaeus was a priest of the Church of Lyon. The clergy of that city, many of whom were suffering imprisonment for the faith, sent him to Rome in 177, with a letter to Pope Eleutherius concerning the heresy of Montanism. While Irenaeus was in Rome, a persecution took place in Lyon. Returning to Gaul, he succeeded the martyr Saint Pothinus and became the second bishop of Lyon.

Irenaeus was a student of Polycarp, who was said to have been tutored by John the Apostle. The writings of Irenaeus entitle him to a high place among the fathers of the Church, for they not only laid the foundations of Christian theology but, by exposing and refuting the errors of the gnostics, they delivered the Catholic Faith from the real danger of the doctrines of those heretics.

Before Irenaeus, Christians differed as to which gospel they preferred. The Christians of Asia Minor preferred the Gospel of John. Irenaeus asserted that all four of the Gospels, Matthew, Mark, Luke, and John, were canonical scripture.

The central point of Irenaeus's theology is the unity and the goodness of God, in opposition to the Gnostics' theory of God. Irenaeus used the Logos theology that he inherited from Justin Martyr. According to Irenaeus, the high point in salvation history is the advent of Jesus. For Irenaeus, the Incarnation of Christ was intended by God, even before He determined that humanity would be created.

Irenaeus sees Christ as the new Adam, who systematically undoes what Adam did: thus, where Adam was disobedient concerning God's edict concerning the fruit of the Tree of Knowledge of Good and Evil, Christ was obedient even to death on the wood of a tree. Irenaeus is the first to draw comparisons between Eve and Mary, contrasting the faithlessness of the former with the faithfulness of the latter.

The date of death of St. Irenaeus is not known, but it is believed to be in the year 202. The bodily remains of St. Irenaeus were buried in a crypt under the altar of what was then called the church of St. John, but was later known by the name of St. Irenaeus himself. This tomb or shrine was destroyed by the Calvinists in 1562, and all trace of his relics seems to have perished.

Pope Francis declared Irenaeus the 37th Doctor of the Church on 21 January 2022.[12]

Other Saints of the Day
1. Saint Plutarch
2. Saint Benignus
3. Saint Argymirus
4. Saint Crummine
5. Saint Vincenza Gerosa

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