VATICAN CITY — Pope Francis on Friday officially declared St. Irenaeus of Lyon as the 37th Doctor of the Church, with the title “Doctor Unitatis” (“Doctor of Unity”).

The saint is the first martyr to be declared a Doctor of the Church.

“May the doctrine of such a great Master encourage more and more the path of all the Lord's disciples towards full communion,” the Pope wrote in a decree signed on Jan. 21.

The Pope signed the decree mid-way through the Week of Prayer for Christian Unity, taking place on Jan. 18-25.

“St. Irenaeus of Lyon, who came from the East, exercised his episcopal ministry in the West: he was a spiritual and theological bridge between Eastern and Western Christians,” Pope Francis wrote.

“His name, Irenaeus, expresses that peace which comes from the Lord and which reconciles, restoring unity.”

St. Irenaeus is a 2nd-century bishop and writer revered by both Catholics and Orthodox Christians and known for refuting the heresies of Gnosticism with a defense of both Christ’s humanity and divinity.

While some of St. Irenaeus’ most important writings have survived, the details of his life are not as well preserved. He was born in the Eastern half of the Roman Empire, likely in the coastal city of Smyrna, in what is now Turkey, around the year 140 A.D.

As a young man, he heard the preaching of the early Christian bishop St. Polycarp, who had been personally instructed by the Apostle John. Irenaeus became a priest, serving the Church in the region of Gaul, in what is now France, during a difficult period in the late 170s.

During this time of state persecution and doctrinal controversy, Irenaeus was sent to Rome to provide Pope St. Eleutherius with a letter about the heretical movement known as Montanism.

After returning to Lyon, Irenaeus became the city’s second bishop, following the martyrdom of his predecessor St. Pothinus.

In the course of his work as a pastor and evangelist, the second bishop of Lyon came up against heretical doctrines and movements that insisted that the material world was evil and not part of God’s original plan.

Irenaeus recognized this movement, in all its forms, as a direct attack on the Catholic faith. He rebutted the Gnostic errors in his lengthy book Against Heresies, which is still studied today for its historical value and theological insights.

A shorter work, the Proof of the Apostolic Preaching, contains Irenaeus’ presentation of the Gospel with a focus on Jesus Christ’s fulfillment of Old Testament prophecies. Several of his other works are now lost, though a collection of fragments from them has been compiled and translated.

Irenaeus died in Lyon around 202, when Emperor Septimus Severus ordered the martyrdom of Christians.

The U.S. bishops voted in 2019 in favor of having St. Irenaeus named a Doctor of the Church at the request of Cardinal Philippe Barbarin, the then archbishop of Lyon, and sent their approval to the Vatican for the pope’s consideration.

Pope Francis previously declared St. Gregory of Narek, a 10th-century Armenian monk, a Doctor of the Church in 2015.

Benedict XVI named Sts. John of Avila and Hildegard of Bingen Doctors of the Church in 2012.

Seventeen of the 36 figures declared Doctors of the Church by the Catholic Church lived before the Great Schism of 1054 and are also revered by Orthodox Christians.