Saint Irenaeus was a Greek Bishop known for his role in guiding and expanding Christianity throughout the south of France and was a disciple of Saint Polycarp, the last known living connection with the Apostles.
Saint Irenaeus was a Greek man that was born around the year 130 AD and grew up in the ancient town of Smyrna (now Izmir Turkey) and unlike many of his contemporaries he was born into a Christian family rather than someone who converted later as an adult. Her served as a priest at the Church in Lyon during the persecution of Marcus Aurelius. It was during this time he travelled to Rome with a letter to Pop Elutherius regarding the heresy of Montanism and returned to Gaul soon afterwards to find the Bishop there, Saint Pothinus had been martyred. Saint Irenaeus followed him as the second Bishop of Lyon. Upon his new consecration as Bishop, Saint Irenaeus began to create many critically important writings , the most famous being the Adversus haereses (Against Heresies). He died sometime during the beginning of the third century and was martyred, but very little else is known regarding his later life, only that he was then buried in the Church or Saint John after his martyrdom.
He tackled the Valentinian Gnostics and their predecessors in his Against Heresies and laid out several writings in the first three books laying out reasons why their doctrines are false. The Fourth book consists of sayings from Jesus and the unity of the Old Testament and Gospels while the fifth book focuses more on the letters of Saint Paul. In a famous countering to the Gnostic teachings Saint Irenaeus significantly developed Saint Paul's presentation of Jesus Christ as the New Adam. He described that how Christ's taking human flesh saves humanity in a statement closely resembling Romans 5:19 -
"For as by the disobedience of the one man who was originally moulded from virgin soil, the many were made sinners, and forfeited life; so was it necessary that, by the obedience of one man, who was originally born from a virgin, many should be justified and receive salvation."
Saint Irenaeus spent time considering the mystic number 666 stating:
"But knowing the sure number declared by Scripture, that is six hundred sixty and six, let them await, in the first place, the division of the kingdom into ten; then, in the next place, when these kings are reigning, and beginning to set their affairs in order, and advance their kingdom, [let them learn] to acknowledge that he who shall come claiming the kingdom for himself, and shall terrify those men of whom we have been speaking, have a name containing the aforesaid number, is truly the abomination of desolation."
Saint Irenaeus is honored with the titles Bishop, Martyr, and Teacher of the Faith with a feast day celebrated in the Roman Catholic Church of June 28th (August 23rd in the Eastern Orthodox Church).
An online version of Against Heresies may be found here : https://www.newadvent.org/fathers/0103.htm