4 But God is first of all glorified here, while He is being made known to men by word of mouth, and preached through the faith of believers. Wherefore, He says, “I have glorified You on the earth: I have finished the work which You gave me to do.” He does not say, You ordered; but, “You gave:” where the evident grace of it is commended to notice. For what has the human nature even in the Only-begotten, that it has not received? Did it not receive this, that it should do no evil, but all good things, when it was assumed into the unity of His person by the Word, by whom all things were made?
But how has He finished the work which was committed unto Him to do, when there still remains the trial of the passion wherein He especially furnished His martyrs with the example they were to follow, whereof, says the apostle Peter, “Christ suffered for us, leaving us an example, that we should follow His steps:” but just that He says He has finished, what He knew with perfect certainty that He would finish? Just as long before, in prophecy, He used words in the past tense, when what He said was to take place very many years afterwards: “They pierced,” He says, “my hands and my feet, they counted all my bones;” He says not, They will pierce, and, They will count.
And in this very Gospel He says, “All things that I have heard of my Father, I have made known unto you;” to whom He afterward declares, “I have yet many things to say unto you, but you cannot bear them now.” For He, who has predestinated all that is to be by sure and unchangeable causes, has done whatever He is to do: as it was also declared of Him by the prophet, “Who has made the things that are to be.”
Source: Tractates on the Gospel of John (New Advent)