John 17:4
“I have glorified You on the earth.” Well said He, “on the earth”; for in heaven He had been already glorified, having His own natural glory, and being worshipped by the Angels. Christ then speaks not of that glory which is bound up with His Essence, (for that glory, though none glorify Him, He ever possesses in its fullness,) but of that which comes from the service of men. And so the, “Glorify Me,” is of this kind; and that you may understand that He speaks of this manner of glory, hear what follows.
“I have finished the work which You gave Me that I should do it.”
And yet the action was still but beginning, or rather was not yet beginning. How then said He, “I have finished”? Either He means, that “I have done all My part”; or He speaks of the future, as having already come to pass; or, which one may say most of all, that all was already effected, because the root of blessings had been laid, which fruits would certainly and necessarily follow, and from His being present at and assisting in those things which should take place after these. On this account He says again in a condescending way, “Which You gave Me.” For had He indeed waited to hear and learn, this would have fallen far short of His glory. For that He came to this of His own will, is clear from many passages. As when Paul says, that “He so loved us, as to give Himself for us”; and, “He emptied Himself, and took upon Him the form of a servant”; and, “As the Father has loved Me, so have I loved you.”
Source: Homilies on the Gospel of John (New Advent)