Towards the end of the second century when gnostic sects endeavoured to undermine the basis of the Christian religion, God raised up St. Irenaeus to oppose them. "He granted him the grace to destroy the heresies by the truth of his doctrine" (Collect).
Succeeding St. Pothinus in the See of Lyons in 177, St. Irenaeus "preached in season and out of season" as St. Paul prescribes (Epistle) and constituted himself defender of Christ (Gospel) and of His Spouse. "The Church," he declares, "disseminated throughout the world, to the extremities of the earth, professes the faith she has received from the apostles, who themselves received it from the Son of God." This Church has its centre at Rome. "With her, every church must be in agreement because of her primacy: for through the succession of Roman pontiffs the apostolic tradition of the Church has come down to us ."
An ardent apologist, St. Irenaeus was also a profound theologian. He has been called the father of the Catholic theology and the golden link binding the spirit of the Gospel to the doctrine of the Fathers. With his ears still full of the last echoes of apostolic teaching (Alleluia), he was the first to write a reasoned summary of our faith. His treatise: "False doctrine unmasked and refuted" also called "Against heresies" gave the death blow to the gnostic heresy.
St. Jerome gives him the glorious title of Martyr. He died, as is believed, during the persecution of Septimus Severus in 202. Benedict XV extended his feast to the universal Church.
Lex veritatis fuit in ore ejus, et iniquitas non est inventa in labiis ejus: in pace, et in aequitate ambulavit mecum, et multos avertit ab iniquitate. * Attendite, popule meus, legem meam: inclinate aurem vestram in verba oris mei.
The law of truth was in his mouth, and iniquity was not found in his lips: he walked with me in peace, and in equity, and turned many away from iniquity. * Attend, O My people, to My law: incline your ear to the words of My mouth.
Malachias 2:6 and Psalm 77:1 from the introit of mass)
Deus, qui beato Irenaeo Martyri tuo atque Pontifici tribuisti, ut et veritate doctrinae expugnaret haereses, et pacem Ecclesiae feliciter confirmaret: da, quaesumus, plebi tuae in sancta religione constantiam; et pacem tuam nostris concede temporibus.
O God, who didst grant that blessed Irenaeus, Thy martyr and bishop, should both overcome heresy by the truth of his doctrine, and establish peace in Thy Church: give unto Thy people, we beseech Thee, constancy in their holy religion, and grant us Thy peace in our days.
From the Catholic Encyclopaedia: http://www.newadvent.org/cathen/08130b.htm