39 For this reason, having already so well prepared their minds for the understanding of this belief, the Lord follows up the words, And this is eternal life, that they should know You, the only true God, and Him Whom You sent, even Jesus Christ, with a reference to the obedience displayed in His incarnation, I have glorified You on the earth, I have accomplished the work which You gave Me to do. And then, that we might know the reward of His obedience, and the secret purpose of the whole divine plan, He continued, And now, O Father, glorify Thou Me with Your own self, with the glory which I had with You before the world was. Does any one deny that Christ remained in the nature of God or believe Him separable and distinct from the only true God?
Let him tell us what is the meaning of this prayer. And now, O Father, glorify Thou Me with Your own self. For what purpose should the Father glorify Him with His own self? What is the signification of these words? What follows from their signification? The Father neither stood in need of glory, nor had He emptied Himself of the form of His glory. How should He glorify the Son with His own self, and with that glory which He had with Him before the world was made? And what is the sense of which He had with Him? Christ does not say, “The glory which I had before the world was made, when I was with You,” but, The glory which I had with You.
When I was with You would signify, “when I dwelt by Your side:” but which I had with You teaches the Mystery of His nature. Further, Glorify Me with Yourself is not the same as “Glorify Me.” He does not ask merely that He may be glorified, that He may have some special glory of His own, but prays that He may be glorified of the Father with Himself. The Father was to glorify Him with Himself, that He might abide in unity with Him as before, since the unity with the Father's glory had left Him through the obedience of the Incarnation.
And this means that the glorifying should reinstate Him in that nature, with which He was united by the Mystery of His divine birth; that He might be glorified of the Father with Himself; that He should resume all that He had had with the Father before; that the assumption of the servant's form should not estrange from Him the nature of the form of God, but that God should glorify in Himself the form of the servant, that it might become for ever the form of God, since He, Who had before abode in the form of God, was now in the form of a servant. And since the form of a servant was to be glorified in the form of God, it was to be glorified in Him in Whose form the fashion of the servant's form was to be honoured.
Source: On the Trinity (New Advent)