It may help to put today's Gospel into perspective if we recognize that the events of our Lord's transfiguration occurred very briefly after that gathering of our Lord and His apostles in Caesarea Philippi, during which our Lord asked them "Who do men say the Son of Man is?" and Peter correctly identified Him as "the Son of the living God." Our Lord was pleased with Peter because his answer demonstrated that he had been chosen by God the Father to know things that were above the knowledge of ordinary "flesh and blood" men or women. Indeed, it was at this time that our Lord announced that He was going to found His Church upon Peter, whom He identified as the "rock" or "foundation stone" (Mt. xvi).
Yet only a little while later, our Lord announced that the time was drawing near for that very same Son of Man to go up to Jerusalem to be put to death by "the elders and scribes and chief priests ... and on the third day rise again." And this time, Peter's reaction was not one that had been revealed by God to be appropriate -- rather it was the reaction of simple human terror, perhaps mixed with indignation: "Far be it from me, Lord. This will never happen to Thee." Peter seemed to be saying that he wasn't going to allow anything like the Crucifixion to take place -- not if he could do anything about it.
Our Lord's response was quite something -- particularly when we consider that He was addressing the man whom He had just made the first Pope: "Get thou behind Me, Satan; thou art a scandal to Me. Thou dost not mind the things of God, but those of men!" "Satan!" He called the first Pope a devil!
Peter apparently understood the theoretical things, like Christ being the Son of the living God -- but he seemed to miss the practical application of this theoretical truth -- that no matter how splendid it might have been to be Christ or one of His apostles, there were difficult practical things to be done before they could all sit back and enjoy their rewards in the kingdom of heaven. In fact, it was not enough for the apostles to simply follow along as mere spectators to witness the events that would soon take place in Jerusalem. To the contrary, our Lord told them (as He tells us) that anyone who wants to come after Him must pick up his own cross and follow Him.
Whether or not Peter was convinced, remains to be seen. Jesus spoke about losing one's life in order to gain it, and the possibility, even, of losing one's soul in exchange for a lot of useless things. And, before He went away to pray, our Lord said something cryptic -- that some of those who had been standing there would not taste death until they had "seen the Son of Man in His kingdom." The Gospel account takes up once again six days later with the passage we just read this morning (Mt. xvii). Peter, together with James and John, accompanied our Lord to a high mountain top (a place called Mount Thabor). And, as we read, our Lord was spectacularly changed in His appearance, becoming as bright as the sun and as the reflection of the sun on the whitest snow. If that wasn't enough, they were joined by the two great fathers of the Old Testament, Moses and Elias -- representing the Law and the Prophecy of God. As He had predicted, not quite a week ago, they were seeing our Lord in His kingdom! (Or, at least in the material glory that He would have with His elect in heaven -- for no one could actually behold His divine glory and yet live.)
Peter was quick to see an opportunity. This certainly beat going to Jerusalem and getting crucified. If this was His kingdom, why would Jesus ever want to leave it? And, never one to hesitate, Peter spoke up: "Lord, why don't we put up some shelter and just stay here on your lovely mountain top; we could put up a few tents, and Moses and Elias could stay too!"
But just about as soon as Peter got those words out of his mouth, an even brighter glory closed in, a sort of luminous cloud that overshadowed everything on the mountain top. And then they heard the very voice of God the Father Himself: "This is My beloved Son, in whom I am well pleased; hear Him." You can almost see Peter on the ground, hugging the top of the mountain as though he was afraid that he would fall off and go flying through space! He had heard the voice of God and seen the reflection of His glory -- he had experienced about as much of God the Father as it was possible for any living man.
One might expect that, from that moment, Peter would have no more hesitation about the need to carry the cross with our Lord. But that would be expecting too much. Peter remained a human being, possessed of all of the fears and desires that we human beings have. God did not take away his free will and make him something less than a man. Peter would still question our Lord: "We have given up every thing to follow Thee; what then shall we have?" (Mt. xix). ("Everyone who has left house, or brothers, or sisters, or father, or mother, or wife, or children, or lands, shall receive a hundredfold and possess life everlasting.") Peter would draw a sword and maim the servant of the high priest in order to avoid the capture of Jesus in the garden of Gethsemane (Jn. xviii). In spite of hearing the voice of God on the mountain, Peter was still human enough in his fears, even to deny that he knew the Christ (Mt. xxvi).
Yet, Peter would persevere in the end. God never took away Peter's free will, but He did strengthen it, together with his intellect, as well. "Grace," we say, "perfects nature." Not many days hence, after the horror of the Crucifixion and the glory of the Resurrection, would come the desent of the Holy Ghost. And this very same Peter would be out there preaching to three-thousand converts on Pentecost Sunday. This very same Peter would spend time, first in the jails of Jerusalem, and later in the jails of Rome. He would plant the Catholic Church at the very seat of the Roman Empire before dying. He would even have the strength and the courage to tell his executioners how they ought to crucify him -- upside down, so that no one might mistake the servant for the master. Grace would perfect his frail human nature. Saint Paul would write down the words that God spoke to him in the same context: "My grace is sufficient for thee; for strength is made perfect in weakness" (2 Cor. xii).
We are not really much different, you and I. We have human free will, every bit as much as had Peter. We have our own fears and desires -- needs and emotions that keep us from seeing things all the time from God's perspective. But we also have some of the opportunities that Peter had. The Lenten observance may not require us to give up our wife, or our children, or our home as the apostolate required of Peter -- but "the fasting of our body" still will help to "curb our vices, elevate our minds, and bestow virtue and reward." Perhaps even more importantly, we have the opportunity to have our "nature perfected by grace" -- just as Peter did -- living daily in the company of our Lord, and being perfected by the Sanctifying Grace of His Sacraments.
Make the best of the remaining days of Lent -- Mass and Communion as often as possible, the Rosary, the Stations of the Cross, the spiritual reading, and doing good for other people. We are all human -- but "Grace will perfect nature"; God's strength will be perfected in our weakness.