1. Humanity Perfected
I think it meant two things in one. It certainly represented and set forth the absolute perfecting of His humanity. Here He has reached the point of His own personal perfecting as a Man. This glorifying, this transfiguring, was Heaven's testimony to His utter and perfect sinlessness as a Man: that in all respects, whether of hell's assaults and temptations and subtleties and efforts, or men's hatred, malice, trickery and what not, He had triumphed, completely triumphed. If we were to analyze it, we should have to look at the word sin. But we can say this, that the sum of sin, from the beginning in the garden to the end, is unfaithfulness to God - a breach of fellowship with God through mistrust. That is the very core of sin. Everything was concentrated upon Him, from every realm, if by some means, in some way, a breach could be made between Him and God That would be sin.
But in His case it never happened. He met it all and triumphed. The first Adam failed, and all his seed have been involved - but here is a Man perfected. Humanity that God intended is here achieved and realized, and is therefore glorified. So far as He was concerned, that was the first meaning: Sin, with all its horrible entail, has been completely defeated in and by this Man; and therefore death must go. There can be no death, for death is the result of sin. If Adam had never sinned, he would never have died. This One never sinned: He could not die - He could only be glorified!
2. The Return of His Glory
There is another aspect as to its meaning to Him. I think it is quite clear that the Lord Jesus carried in His heart a great longing and a prayer for the glory that He once had. This is where I think John touches this matter very closely. In the seventeenth chapter of his gospel, he records that great prayer of the Lord Jesus: "Father, glorify Thou Me with the glory which I had with Thee before the world was" (vs. 5). That opens a window and lets us see that the Lord Jesus had a consciousness of His eternal glory past: He carried it with Him; He knew about it - marvelous thought! - and that the consciousness of that former glory was ever prompting Him to pray toward, long toward, the day when He would return to it and it would return to Him. "Father, glorify Thou Me with the glory which I had with Thee before the world was."
The Mount of Transfiguration had become an answer to His heart's prayer and cry and longing - at least a touch of it. A fleeting touch, but for Him it was one of those things which perhaps you know a little about in your Christian life. The Lord just does something - it passes, but you know by it that you have been heard; you know that there is sympathy in the Father's heart for your need and situation. It may only last for a day, or a night, for an hour, or for a little while, and then pass, because the end of the road is not yet; the eternal glory has not yet come; but the touch by the way is something that carries us on. We know the Lord has heard; we know the Lord has taken account of that inner cry and longing, and has given us a token of His sympathy. It was like that with the Lord Jesus - the answer to His own cry.
(continued with # 4 - "3. The Offset to the Cross")