S. John the Apostle, the son of Zebedee and Salome, the brother of S. James the Greater, wrote this gospel in Asia in the Greek language, toward the end of his life, after his return from exile on Patmos, where he had written the Apocalypse, as I showed in the preface to the Apocalypse.

His reasons for writing were two. The first was that he might confute Ebion and Cerinthus, heretics who at that time were already beginning to agitate, who denied Christ’s Divinity, and taught that He was a mere man. The second was to supply the things that Matthew, Mark and Luke had omitted concerning Christ. Hence S. John records at length what Christ did during the first year of His preaching, which the other three had for the most part passed over.

Listen to S. Jerome in his preface to S. Matthew. “Last was John, the Apostle and Evangelist, whom Jesus loved the best, who lay upon the Lord’s bosom, and drank of the purest streams of His doctrines, and who alone merited to hear from the cross, ‘Behold thy mother.’”

When he was in Asia, at a time when the seeds of the heresies of Cerinthus, Ebion and the rest were sprouting, who denied that Christ had come in the flesh, those whom in his epistle he calls antichrists, and whom the Apostle Paul frequently refutes, he was constrained by well nigh all the bishops who were at that time in Asia, and by the delegates of many other churches, to write more profoundly about the deep things of the Divinity of our Savior, and to ‘break through,’ so to speak, to the Word of God by a temerity which is not so daring as it is felicitous. Whence also we are told in ecclesiastical history that when he was urged by the brethren to write, he replied that he would do so, on condition that they should all fast, and pray to God in common. When the fast was ended, being filled with the power of revelation, he burst forth with the preface coming straight from heaven, In the beginning was the Word, and the Word was with God, and the Word was God. The same was in the beginning with God.

Therefore S. John wrote his gospel following a general fast in the Church. Others add that it was preceded by thundering and lightning, as though he had been another Moses, who thus received the law from God in the midst of thunder, lightning and an earthquake (Exodus 19). For Christ nicknamed him Boanerges, that is, “son of thunder,” and as such he thundered the new beginning of the new law: In the beginning was the Word. Thus Baronius, citing Metaphrastes and other authors in volume one, near the beginning of the entry for the year of our Lord 99, where he likewise shows that John wrote his gospel in the year of Christ 99, or sixty-six years after the ascension, which was twenty seven years after the destruction of Jerusalem by Titus. This was the first year of the reign of Nerva, in which the Christian faith was now growing strong throughout the provinces, and the Church was becoming renowned for the great number of her believers and the greater virtue and strength of her martyrs.

As then Isaias surpassed all the rest of the prophets in sublimity, so did John the other evangelists. And therefore, although he is last in time, he is first in dignity and perfection. Thus he is compared to an eagle flying above all other birds (Ezechiel 1; Apocalypse 4). This, his dignity, sublimity and special excellence, and consequently his obscurity, may be considered under three heads. First, his matter and argument. S. John alone of set purpose treats of the Divinity of Christ, of the origin, eternity, and generation of the Word, of the spiration of the Holy Spirit, of the Most Holy Trinity, of the unity of the Godhead, and of the divine relations and attributes. Matthew, Mark, and Luke are concerned with the mysteries and actions of Christ’s humanity. This is why the fathers derive almost all their arguments against the Arians, Servetians, Nestorians, Eutychians, and such like heretics from S. John; whereas the scholastic doctors drew from him all their material concerning the Holy Trinity and about God, One and Triune.

The second is the order of time. We know that the Church, like the dawn and the sunrise, gradually advanced by the succession of time to the perfect day of the knowledge of the mysteries of the Faith. Thus the sacred writers of the New Testament, the Apostles and Evangelists, write far more clearly and splendidly concerning them than do Moses and the prophets of the Old Testament. S. John was the last of all, and his gospel was his last work, as the fathers and commentators teach. With this holy gospel, therefore, John put the finishing touch and crown on the gospels and sacred books.

The third is the author. S. John alone merited and won the laurels of all the saints. For he is called, and is indeed, a theologian, or rather the prince of theologians. He is a prophet, an Apostle, an Evangelist; he is a priest, a high priest, a hierarch; he is both virgin and martyr. (Because he was, and remained, a virgin, he was neither burnt nor harmed when he was immersed in boiling oil.) That John was a theologian is clear from the Apocalypse, which in Greek is entitled The Apocalypse of Saint John the Theologian. That he is a prophet in the Apocalypse no one can overlook: he is an Evangelist in his gospel: an Apostle in the three canonical epistles which he composed. That he was a virgin and always remained so is asserted by all the ancient writers, in the earliest instance by Tertullian (lib. de Monogamia) and by S. Jerome (lib. 1 contra Jovin.). Thus by the merits of his other virtues, and especially his virginity, he became the disciple whom Jesus loved, the “Benjamin” of Christ, who at the last supper leaned on our Lord’s breast; to him as to a virgin, therefore, He, while He was dying, commended His virgin mother. For blessed are the clean in heart, for they shall see God, as the truth itself declares (Matth. 5:8). They shall see Him, I say, in this life by faith and contemplation, and in the next by appearance and vision.

The Only-Begotten Son, who is in the bosom of God the Father, made known to S. John, His most chaste and beloved friend, who reclined upon His breast, the secrets and mysteries of the Divinity, which had been hidden from the foundation of the world. John hath declared the same to us, as a son of thunder, thundering and lightening the whole world with the Divinity of the Word, as with a flaming thunderbolt, and with the fire of love he hath inflamed it. Let that one speech of Christ, His longest and His last, bear witness, which He made after supper (ch. 13 ff.), which breathes of nothing but the ardor of divine love. “John,” says S. Epiphanius (haer. 73), “a true son of thunder by his own great eloquence, as though from the clouds of wisdom’s mysteries, has imparted to us a pious knowledge of the Son.” S. Jerome (epist. 85 ad Evagrium) writes, “The trumpet sounds, the son of thunder, whom Jesus loved more dearly, who on the Savior’s breast drank from the living waters of doctrine.”

You can read more about him in S. Cyril, S. Augustine, and S. Chrysostom (proaemio in Joannem). Indeed, S. Chrysostom dares to say that S. John in his gospel hath taught the angels the secrets of the incarnate Word, such as before they knew not, and that therefore he is the doctor of the cherubim and the seraphim. He proves this from Paul’s words in Ephesians 3:10, That the manifold wisdom of God may be made known to the principalities and powers in heavenly places through the Church. “If, therefore,” says Chrysostom, “the principalities and powers, the cherubim and seraphim, have learned these things through the Church, it is very evident that the angels listen to him with the deepest attention. Not slight therefore is the honor which we gain in that the angels are our fellow-disciples in the things that they knew not.”

S. Gregory of Nyssa, the brother of S. Basil, relates in his Life of S. Gregory Thaumaturgus, that the latter, because he had been a disciple of Origen, and was about to be created the bishop of NeoCæsarea, at the time when the followers of Origen were scattering his errors abroad and sowing the seeds of Arianism, began to waver about doctrine and the correct manner of teaching; therefore he began to call humbly upon God and His mother for help and light, and not in vain. In a dream the Blessed Virgin appeared to him with S. John, and she gave him S. John, himself, as his teacher. Thus S. John handed down to S. Gregory Thaumaturgus the Creed which the Fifth Ecumenical Council and the entire Eastern Church proclaims. By means of this Creed he pellucidly explains and treats of many divine mysteries of his gospel, especially concerning the Word who is consubstantial with the Father. This Creed is found in Baronius and in others authors who have written the Life of the Thaumaturgus. Let us humbly beg for the same grace from both saints. I have said more about S. John in the preface to the Apocalypse and the First Epistle of S. John.

Lastly, to S. John Chrysostom, while he was living in the monastery, appeared S. Peter, offering him the keys, and S. John, handing him the gospel book; and just as S. Peter gave him the authority of a bishop [Pontificem], S. John made him a doctor and a heavenly preacher; hence he deserved the name Chrysostom, i.e., “golden mouth.” This is related in his Life.