Christ is the origin of grace. John 1:16-17: ‘And of his fullness we have all received, grace for grace. For the Law was given through Moses: grace and truth came through Jesus Christ.’ In Christ a twofold nature is to be considered, a divine, and the Gospel of John is chiefly concerned with this, hence he begins, ‘In the beginning was the Word and the Word was with God, and the Word was God.’ And a human, and the other Gospels treat chiefly of this, and they are distinguished according to the threefold dignity that belongs to the man Christ. With respect to his royal honour, Matthew speaks. Hence in the beginning of his Gospel he shows that Christ descended from kings and was adored by the Magi kings. With respect to his prophetic honour, Mark speaks, hence he begins with the preaching of the Gospel. With respect to his priestly dignity, Luke speaks, and he begins with the temple and the priesthood and ends his Gospel in the temple, and frequently returns to the temple, as the Gloss says about Luke 2.46: ‘And they found him sitting in the temple in the midst of the teachers.’
In another way, Matthew might be said to speak of Christ chiefly with respect to the mystery of the Incarnation, and thus he is depicted in the figure of a man. Luke, with respect to the mystery of the Passion, and therefore he is depicted as a bull, which is an animal to be immolated. Mark, with respect to the victory of the Resurrection, and thus he is depicted as a lion. But John, who soars to the heights of his divinity, is depicted as an eagle.
Thomas Aquinas, Commendatio Sacrae Scripturae