The beginning of the gospel of Jesus Christ, the Son of God.
Jerome, in Prolog - Mark the Evangelist, who served the priesthood in Israel, according to the flesh a Levite, having been converted to the Lord, wrote his Gospel in Italy, shewing in it how even his family benefited Christ. For commencing his Gospel with the voice of the prophetic cry, he shews the order of the election of Levi, declaring that John the son of Zachariah was sent forth by the voice of an angel, and saying, "The beginning of the Gospel of Jesus Christ, the Son of God."
Pseudo-Jerome - The Greek word 'Evangelium' means good tidings, in Latin it is explained, 'bona annunciatio,' or, the good news; these terms properly belong to the kingdom of God and to the remission of sins; for the Gospel is that by which comes the redemption of the faithful and the beatitude of the saints.
But the four Gospels are one, and one Gospel in four. In Hebrew, His name is Jesus, in Greek, Soter, in Latin, Salvator; but men say Christus in Greek, Messias in Hebrew, Unctus in Latin, that is, King and Priest.
Bede, in Marc., i, 1 - The beginning of this Gospel should be compared with that of Matthew, in which it is said, "The book of the generation of Jesus Christ, the Son of David, the Son of Abraham." But here He is called "the Son of God."
Now from both we must understand one Lord Jesus Christ, Son of God, and of man. And fitly the first Evangelist names Him "Son of man," the second, "Son of God," that from less things our sense may by degrees mount up to greater, and by faith and the sacraments of the human nature assumed, rise to the acknowledgment of His divine eternity.
Fitly also did He, who was about to describe His human generation, begin with a son of man, namely, David or Abraham. Fitly again, he who was beginning his book with the first preaching of the Gospel, chose rather to call Jesus Christ, "the Son of God;" for it belonged to the human nature to take upon Him the reality of our flesh, of the race of the patriarchs, and it was the work of Divine power to preach the Gospel to the world.
Hilary, de Trin., iii, 11 - He has testified, that Christ was the Son of God, not in name only, but by His own proper nature. We are the sons of God, but He is not a son as we are; for He is the very and proper Son, by origin, not by adoption; in truth, not in name; by birth, not by creation.
- text taken from Catena Aurea - Gospel of Mark by Saint Thomas Aquinas, translated by William Whiston, 1842