13 We must next ascertain what and whence this glorifying is. God, I am sure, is subject to no change; His eternity admits not of defect or amendment, of gain or of loss. It is the character of Him alone, that what He is, He is from everlasting. What He from everlasting is, it is by His nature impossible that He should ever cease to be. How then can He receive glory, a thing which He fully possesses, and of which His store does not diminish; there being no fresh glory which He can obtain, and none that He has lost and can recover?
We are brought to a standstill. But the Evangelist does not fail us, though our reason has displayed its helplessness. To tell us what return of glory it was that the Son should make to the Father, he gives the words: Even as You have given Him power over all flesh, that whatsoever You have given Him He may give it eternal life. And this is life eternal that they should know You, the only true God, and Jesus Christ Whom You have sent. The Father, then, is glorified through the Son, by His being made known to us.
And the glory was this, that the Son, being made flesh, received from Him power over all flesh, and the charge of restoring eternal life to us, ephemeral beings burdened with the body. Eternal life for us was the result not of work done, but of innate power; not by a new creation, but simply by knowledge of God, was the glory of that eternity to be acquired. Nothing was added to God's glory; it had not decreased, and so could not be replenished. But He is glorified through the Son in the sight of us, ignorant, exiled, defiled, dwelling in hopeless death and lawless darkness; glorified inasmuch as the Son, by virtue of that power over all flesh which the Father gave Him, was to bestow on us eternal life.
It is through this work of the Son that the Father is glorified. So when the Son received all things from the Father, the Father glorified Him; and conversely, when all things were made through the Son, He glorified the Father. The return of glory given lies herein, that all the glory which the Son has is the glory of the Father, since everything He has is the Father's gift. For the glory of Him who executes a charge redounds to the glory of Him Who gave it, the glory of the Begotten to the glory of the Begetter.
Source: On the Trinity (New Advent)