3 John, however, may say something more evidently, that our Lord Jesus Christ is God. We may find this in the present passage, that it is perhaps of Him we have been singing, “The Lord reigned over all the earth;” against which they are deaf who imagine that He reigns only in Africa. But let them not suppose that it is not of Christ it is spoken when it is said, “God reigned over all the earth.” For who else is our King, but our Lord Jesus Christ? It is He that is our King.
And what have you heard in the same psalm, in the verse just sung? “Sing praises to our God, sing praises: sing praises to our King, sing praises.” Whom he called God, the same he called our King: “Sing praises to our God, sing praises: sing praises to our King, sing ye praises with understanding.” And that you should not understand Him to whom you sing praises to reign in one part, he says, “For God is King of all the earth.” And how is He King of all the earth, who appeared in one part of the earth, in Jerusalem, in Judea,walking among men, born, sucking the breast, growing, eating, drinking, waking, sleeping, sitting at a well, wearied; laid hold of, scourged, spat upon,crowned with thorns, hanged on a tree, wounded with a spear, dead, buried?
How then King of all the earth? What was seen locally was flesh, to carnal eyes only flesh was visible; the immortal majesty was concealed in mortal flesh. And with what eyes shall we be able to behold the immortal majesty, after penetrating through the structure of the flesh? There is another eye, there is an inner eye. Tobias, for example, was not without eyes, when, blind in his bodily eyes, he was giving precepts of life to his son. The son was holding the father's hand, that the father might walk with his feet, while the father was giving the son counsel to walk in the way of righteousness.
Here I see eyes, and there I understand eyes. And better are the eyes of him that gives counsel of life, than his who holds the hand. Such eyes Jesus also required when He said to Philip, “Am I so long time with you, and you have not known me?” Such eyes He required when He said, “Philip, he that sees me, sees the Father.” These are the eyes of the understanding, these are the eyes of the mind. It is for that reason that the psalm, when it had said, “For God is King of all the earth,” immediately added, “Sing ye praises with understanding.”
For in that I say, “Sing ye praises to our God,” I say that God is our King. But yet our King you have seen among men, as man; you have seen Him suffering, crucified, dead: there was in that flesh something concealed, which you might have seen with eyes of flesh. What was there concealed? “Sing ye praises with understanding.” Do not seek to see with the eyes what is beheld by the mind. “Sing praises” with the tongue, for He is among you as flesh; but because “the Word was made flesh, and dwelt among us,” render the sound to the flesh, render to God the gaze of the mind. “Sing ye praises with understanding,” and you see that the “Word was made flesh, and dwelt among us.”
Source: Tractates on the Gospel of John (New Advent)