Hieromartyr Irenaeus, Bishop of Lyon, was born in 130 in the city of Smyrna. There he received a brilliant education, studying poetry, philosophy, rhetoric and all the other sciences that were considered necessary for a secular youth. His mentor in the truths of Christian teaching was a disciple of the Apostle John the Theologian, St. Polycarp of Smyrna (commemorated on February 23). He also baptized the young man, subsequently ordained him a presbyter and sent him to the Gallic city of Lugdunum (now Lyon in France) to the elderly Bishop Pofin. Soon Saint Irenaeus was given a responsible task: to deliver to the Holy Pope Eleutherius (177-190) a letter from the confessors. 

During his absence, all prominent Christians were imprisoned. In 178, a year after the martyrdom of Bishop Pofin, St. Irenaeus was elected bishop of Lugdunum. "In a short time," St. Gregory of Tours wrote about him, "with his sermon he transformed the whole of Lugdunum into a Christian city!" When the persecution of Christians subsided, the saint expounded the Orthodox doctrine in one of his main works entitled: "The Denunciation and Refutation of false Knowledge," or in short: "Five Books against Heresies." At that time, a number of religious and philosophical gnostic teachings arose. Gnostics (from the Greek word "gnosis" – "knowledge") taught that God cannot incarnate, since matter is imperfect and is the bearer of evil. The Son of God is only an outflow (emanation) of the Deity. Together with Him, a hierarchical series of forces (aeons) emanates from the Deity, the totality of which constitutes a "pleroma", i.e., "fullness". The world was created not by God himself, but by the aeons or the "demiurge", who is below the "pliroma".
In refutation of this heresy of Valentine, St. Irenaeus developed the Orthodox doctrine of salvation. "The word of God, Jesus Christ, our Lord, by His ineffable goodness became what we are, in order to make us what He is...," Saint Irenaeus taught, "Jesus Christ the Son of God, by His excellent love for His creation, descended to be born of a Virgin, through Himself connecting man with God". Through the Incarnation of God, creation becomes conformable and co-corporeal to the Son of God. Salvation consists in the deification of man.
In refutation of the teachings of another heretic, Marcion, who denied the Divinity of the Old Testament, the saint developed the doctrine of a Single Source of the Old and New Testaments: "The same Spirit of God, who through the prophets proclaimed what the coming of the Lord was," the saint wrote. "He preached through the apostles that the fullness of the times of adoption had come and the Kingdom of heaven was at hand."
St. Irenaeus justified the truth of the church's teaching by the continuity of the episcopate, as well as by the fact that the Church is older than all heretics. "Anyone who wants to know the truth should turn to the Church, because the apostles alone told her the Divine Truth. She is the door to life."
Saint Irenaeus had a beneficial influence in the dispute over the celebration of Easter. In the Church of Asia Minor, the tradition of celebrating Holy Easter on the 14th of Nisan was preserved, regardless of what day it would have to be. Holy Pope Victor (190-202) imperiously demanded uniformity, and his harsh demands could cause a split. On behalf of the Gallic Christians, St. Irenaeus wrote to the pope that a schism should not be allowed because of traditions, first of all it is necessary to cherish the church world.
During the reign of Emperor Severus (193-211), Saint Irenaeus was beheaded by the sword for confessing the faith in 202.
The Evangelist Apostle John the Theologian, St. Polycarp of Smyrna and St. Irenaeus of Lyon are three links in an inextricable chain of grace–filled succession that comes from the Very Shepherd Commander of the Lord Jesus Christ. In a very old age, Saint Irenaeus wrote to his friend Florin: "I was a boy when I saw you (Florin) at Polycarp's. I remember what happened then, more than what is happening now. And now I can describe to you the places where Blessed Polycarp used to sit and talk. I can describe his way of life, the appearance of his body and the instructions he gave to the people. The intimate treatment he said he had with John and others who had seen the Lord, and all that he remembered of these words, what he had heard from them about the Lord... I listened to it then, by the grace of God, with zeal and wrote not on paper, but on my heart" (Journal of the Moscow Patriarchate, 1957, No. 9, p. 53).