16 Next let us see what this glory is which the Son expects to receive from the Father; and then our exposition will be complete. The sequel is, I have glorified You on the earth, having accomplished the work which You have given Me to do. And now, O Father, glorify Thou Me with Your own Self with the glory which I had with You before the world was. I have manifested Your name unto men. It is, then, by the Son's works that the Father is glorified, in that He is recognised as God, as Father of God the Only-begotten, Who for our salvation willed that His Son should be born as man, even of a virgin; that Son Whose whole life, consummated in the Passion, was consistent with the humiliation of the virgin birth.
Thus, because the Son of God, all-perfect and born from everlasting in the fullness of the Godhead, had now by incarnation become Man and was ready for His death, He prays that He may be glorified with God, even as He was glorifying His Father on the earth; for at that moment the powers of God were being glorified in the flesh before the eyes of a world that knew Him not. But what is this glory with the Father, for which He looks? It is that, of course, which He had with Him before the world was.
He had the fullness of the Godhead; He has it still, for He is God's Son. But He Who was the Son of God had become the Son of man also, for The Word was made flesh. He had not lost His former being, but He had become what He was not before; He had not abdicated His own position, yet He had taken ours; He prays that the nature which He had assumed may be promoted to the glory which He had never renounced. Therefore, since the Son is the Word, and the Word was made flesh, and the Word was God, and was in the beginning with God, and the Word was Son before the foundation of the world; this Son, now incarnate, prayed that flesh might be to the Father what the Son had been.
He prayed that flesh, born in time, might receive the splendour of the everlasting glory, that the corruption of the flesh might be swallowed up, transformed into the power of God and the purity of the Spirit. It is His prayer to God, the Son's confession of the Father, the entreaty of that flesh wherein all shall see Him on the Judgment-day, pierced and bearing the marks of the cross; of that flesh wherein His glory was foreshown upon the Mount, wherein He ascended to heaven and is set down at the right hand of God, wherein Paul saw Him, and Stephen paid Him worship.
Source: On the Trinity (New Advent)