St. Irenaeus of Lyon is the greatest Church Father of the Second Century. He was a great philosopher, theologian, bishop and martyr. A study of his voluminous writings proves beyond any doubt that that nascent Christian Church had full Catholic orthodoxy.
St. Irenaeus is one of the most important Church Fathers. He is the greatest Catholic apologist of second century Christianity. He also bridges the Catholic traditions of East and West. For these reasons, he is one of the most important figures to study and learn about when one desires to become more familiar with 'early Christianity.' Furthermore, he is remarkable in regards to his absolute catholicity. I actually know of several scholars who came from protestant backgrounds but converted to the True Faith once they discovered, read, and studied St. Irenaeus. You see, it is impossible to read his works and not be convinced that Catholicism as we know it (that is, as per our Tradition) is in fact what the earliest Christians believed and practiced.
St. Irenaeus was born in Asia Minor (modern day Turkey), which at that time was the strongest area of Christianity. St. Paul's letters had been gathered and collected in Ephesus (where St. Timothy was bishop) and St. John had spent much of his life living and preaching in that area. For example, St. John writes his seven letters contained in the Book of the Apocalypse to seven churches in Asia Minor which he helped oversee. Moreover, St. Irenaeus was, spiritually speaking, the grandson of St. John the Apostle. St. Irenaeus was a disciple of St. Polycarp, martyr and Bishop of Smyrna, and St. Polycarp was himself a direct disciple of St. John the Apostle. The martrydom of St. Polycarp is one of the oldest surviving records or early Christian martyrdom and reads very similarly to the pages of the New Testament itself. St. Polycarp also knew St. Ignatius of Antioch very well, another disciple of St. John, a great bishop, a Church Father, and a martyr. St. Ignatius of Antioch's letters, written as he was taken to Rome for his martyrdom in the Coliseum, are another great witness to the faith of the early Christians. Thus, we can confidently assert that St. Irenaeus' Christian 'pedigree' is of the highest excellence.
At some point during the reign of Emperor Marcus Aurelius (161-180 AD), there was a significant migration of laborers from Asia Minor to Gaul. Most historians believe this was due to a phenomenon still common today: men from a heavily populated land (Asia Minor) were looking for work and found it with the imperial Roman government which was constructing new cities and roads far away (in Gaul). In order to provide for the spiritual needs of his people, Irenaeus (then just a priest), chose to migrate with them. This too is something we are familiar with for in nearly every wave of Catholic immigration to this country (be in German, Irish, Italian, etc.), priests who are willing to sacrifice their life and comfort have come with their countrymen to continue to provide for their spiritual needs.
The burgeoning Christian community in Gaul came under fierce attack from the Montanist Heresy. This was a kind of precursor to the more modern Pentecostal Movement. The Montanists were gnostics who believed the Holy Ghost had made Himself incarnate in their leader Montanus and that He spoke through his two 'prophetic' daughters. All three received cult like adulation. They promised great graces to those who followed their secret wisdom and predicted that apocalyptic events were imminent. The claimed to live in the Great Age of the Spirit and only those in union with them would be saved at the Final Judgment, which was just around the corner. They prayed in tongues and shook with unnatural gyrations and spasms as they prayed. They practiced great austerity, mortification and disciplines. They altered the Sacred Scriptures. They claimed to be very spiritual and incorporated mysticism form the East in their ritual practices. They were also in favor of much greater 'active participation' by the laity in the Mass, for according to them all were meant to be filled with the prophetic and priestly spirit of the Holy Ghost. Does any of this sound familiar?
In 177 AD, St. Irenaeus was selected by his community to travel to Rome with a letter for Pope St. Eleuterus detailing the heresy of the Montanists, so that the Church would be able to counter and defeat this growing menace. While Irenaeus was in Rome, a great persecution was unleashed upon the Catholics in Lyons by the Romans and many were martyred. Thus, upon his return, he found a decimated Catholic population shaken and in terror. He was consecrated Bishop of Lyons and set upon the great work of rebuilding and strengthening the Catholic community throughout Gaul. Their faith was being tested as gold in fire by these attacks from religious heretics, state paganism, and the threat of the sword. Yet under St. Irenaeus' episcopal leadership, Catholicism grew and flourished despite these many obstacles.
His greatest work is Adversus Haereses (~180 AD). It includes five volumes and is a vertiable encyclopedia of heresies which plagued the early Church. In this work, he presents a profound, brilliant, scriptural and orthodox defense of the Catholic faith. His scriptural interpretations are particularly enlightening because they follow the kinds of hermeneutics used by St. Peter and St. Paul in their New Testament works (very very different from how modern scholars interpret the Scriptures). Most of the heresies St. Irenaeus describes are Gnostic variations that were fund in Asia Minor, Rome and Gaul. In fact, there are a number of heresies that were so utterly vanquished by the great Church Fathers and saintly popes of the first centuries that the only reason we know of their existence is, not from their own heretical works (which were destroyed), but through the faithful writings of St. Irenaeus.
In 190 AD, St. Irenaeus once more traveled to Rome. This time to meet with Pope St. Victor and to persuade him not to excommunicate the Christians of the East over the Quatrodeciman Controversy. Two apostolic traditions had developed independently as to when to celebrate Easter. In Rome, the tradition of St. Peter was followed which always placed Easter on a Sunday (as we have it to this day). But in Asia Minor, the tradition of St. John was followed, which always placed Easter on the 14th of Nissan - a fixed calendar day which could therefore fall on any day of the week. While the Pope was prepared to excommunicate those who refused to follow the tradition of Rome, St. Irenaeus was able to prevail upon him not to take this drastic action because he was personally able to testify that this was indeed a tradition which stemmed from the Beloved Apostle himself. Here was see St. Irenaeus' great concern and solicitude for the universal Church and great respect for the Apostolic Traditions.
Although his death remains uncertain, most sources report that St. Irenaeus suffered martyrdom in Lyons around 202 AD. Although he had been absent during the first great wave of persecution in his diocese, God chose to have him suffer with his flock in the second wave. With his own life, he gave one final and great witness to the Faith and a testimony to all he had taught and written. The blood of St. Irenaeus joined that of all the other early martyrs in becoming the great seeds from which Catholicism flourished and would eventually dominate all the civilized western world.
This great Church Father was buried in a church in Lyons which was subsequently renamed in his honor. It became a great site of pilgrimages, especially in the middle ages, and numerous miracles were ascribed to his relics. Sadly, this Church was sacked, ravaged and utterly destroyed in 1562 AD by roving bands of violent Protestant Huguenots (French Calvinists). His remains were despoiled and have been lost to subsequent generations of Christians.
St. Irenaeus of Lyons, ora pro nobis!