5 But that you may know that Christ is judge even now, hear what follows: “And if I judge, my judgment is true.” Behold, you have Him as your judge, but acknowledge Him as your Saviour, lest you feel the judge. But why has He said that His judgment is true? “Because,” says He, “I am not alone, but I and the Father that sent me.” I have said to you, brethren, that this holy Evangelist John soars exceedingly high: it is with difficulty that he is comprehended. But we need to remind you, beloved, of the deeper mystery of this soaring.
Both in the prophet Ezekiel, and in the Apocalypse of this very John whose Gospel this is, there is mentioned a fourfold living creature, having four characteristic faces; that of a man, of an ox, of a lion, and of an eagle. Those who have handled the mysteries of Holy Scripture before us have, for the most part, understood by this living creature, or rather, these four living creatures, the four evangelists. They have understood the lion as put for king, because he appears to be, in a manner, the king of beasts on account of his strength and terrible valor.
This character is assigned to Matthew, because in the generations of the Lord he followed the royal line, showing how the Lord was, along the royal line, of the seed of David. But Luke, because he begins with the priesthood of Zacharias, mentioning the father of John the Baptist, is designated the ox; for the ox was an important victim in the sacrifice of the priests. To Mark is deservedly assigned the man Christ, because neither has he said anything of the royal authority, nor did he begin with the priestly function, but only set out with the man Christ.
All these have departed but little from the things of earth, that is, from those things which our Lord Jesus Christ performed on earth; of His divinity they have said very little, like men walking with Him on the earth. There remains the eagle; this is John, the preacher of sublime truths, and a contemplator with steady gaze of the inner and eternal light. It is said, indeed, that the young eagles are tested by the parent birds in this way: the young one is suspended from the talons of the male parent and directly exposed to the rays of the sun; if it looks steadily at the sun, it is recognized as a true brood; if its eye quivers, it is allowed to drop off, as a spurious brood.
Now, therefore, consider how sublime are the things he ought to speak who is compared to the eagle; and yet even we, who creep on the earth, weak and hardly of any account among men, venture to handle and to expound these things; and imagine that we can either apprehend when we meditate them, or be apprehended when we speak.
Source: Tractates on the Gospel of John (New Advent)