12 “These things said Isaiah, when he saw His glory, and spoke of Him.” What Isaiah saw, and how it refers to Christ the Lord, are to be read and learned in his book. For he saw Him, not as He is, but in some symbolic way to suit the form that the vision of the prophet had itself to assume. For Moses likewise saw Him, and yet we find him saying to Him whom he saw, “If I have found grace in Your sight, show me now Yourself, that I may clearly see You;” for he saw Him not as He is.
But the time when this shall yet be our experience, that same Saint John the Evangelist tells us in his Epistle: “Dearly beloved, [now] are we the sons of God; and it has not yet become manifest what we shall be: because we know that, when He shall appear, we shall be like Him; for we shall see Him as He is.” He might have said “for we shall see Him,” without adding “as He is;” but because he knew that He was seen of some of the fathers and prophets, but not as He is, therefore after saying “we shall see Him,” he added “as He is.”
And be not deceived, brethren, by any of those who assert that the Father is invisible, and the Son visible. This assertion is made by those who think that the latter is a creature, and whose understanding runs not in harmony with the words, “I and my Father one.” Accordingly, as respects the form of God wherein He is equal with the Father, the Son also is invisible: but, in order to be seen of men, He assumed the form of a servant, and being made in the likeness of men, became visible to man.
He showed Himself, therefore, even before His incarnation, to the eyes of men, as it pleased Him, in the creature-form at His command, but not as He is. Let us be purifying our hearts by faith, that we may be prepared for that ineffable and, so to speak, invisible vision. For “blessed are the pure in heart; for they shall see God.”
Source: Tractates on the Gospel of John (New Advent)