In his theological masterpiece Human Nature in Its Fourfold State, Thomas Boston explained what he believed the radiance and glory of unfallen Adam in the Garden before the fall would have been like. He wrote:

Man was then a very glorious creature. We have reason, to suppose, that as Moses' face shone when he came down from the mount, so man had a very lightsome and pleasant countenance, and beautiful body, while as yet there was no darkness of sin in him at all. But seeing God Himself is glorious in holiness, (Exod. 15:11) surely that spiritual comeliness the Lord put upon man at his creation made him a very glorious creature. O! how did light shine in his holy conversation, to the glory of the Creator! while every action was but the darting forth of a ray and beam of that glorious, unmixed light, which God had set up in his soul; while that lamp of love, lighted from Heaven, continued burning in his heart, as in the holy place; and the law of the Lord, put in his inward parts by the finger of God, was kept by him there, as in the most holy: There was no impurity to be seen without; no squint look in the eyes, after any unclean thing; the tongue spoke nothing but the language of Heaven: And, in a word. The King's son was all glorious within, and his clothing of wrought gold.1

We are all too painfully aware that this glory was lost when Adam fell. Now man's countenance--which was once radiant--is often seen as downcast and tired, anxious and disturbed. Even the outward appearance of man suggests something of the loss of the glory that we had in Paradise. Understanding this helps us better grasp the significance of the account of the sermon on the Mount when the face of the Son of God, the second Adam, shone like the sun and His clothes became bright and glistening (Matt. 17:2; Luke 9:29). While the glory in which Adam was enshrouded--and that emanated from Moses' face--was merely a reflected glory, the glory of the second Adam was the transforming glory inherent to His Divine nature. It was not reflected. It shone forth from within. It is the glory that He communicates to His people who trust Him for salvation. The apostle Paul refers to this transforming glory when he writes, "But we all, with unveiled face, beholding as in a mirror the glory of the Lord, are being transformed into the same image from glory to glory, just as by the Spirit of the Lord" (2 Cor. 3:18). When Peter, James and John were with Him on the Mount, they were enveloped in glory. Moses and Elijah also appeared in glory. Everyone with Jesus appears in glory. Jonathan Edwards helpfully explained the nature of the glory that shone forth from the Son of God at the Transfiguration when he wrote:

The glory that Christ then appeared in was so divine and admirably excellent, and had such a bright and evident appearance of divinity, such an admirable and ineffable semblance of the infinitely glorious perfection of God, his awful majesty, his purity, and infinitely sweet grace and love, that evidently denoted him to be a divine person. The Apostle says, "He received from the Father honor and glory." The term is doubled and varied, thus to signify the exceeding excellency of the glory.

There was doubtless an inward sight, or lively sense of heart, of Christ's spiritual glory that accompanied Peter's sight of the visible glory of Christ. There was an ineffable beauty, majesty, and brightness in his countenance that held forth and naturally represented the excellencies of his mind, his holiness, his heavenly meekness, and grace, and love, and that majesty that spake his union with the deity, and by the influence of the Spirit of God accompanying, excited in Peter and the other two that were with him a great sense of those perfections, and their immense excellency, adorableness, and sweetness. And the Spirit of God doubtless accompanied the word of God that Peter and the others then heard, so that that word was spiritually understood and believed, so that Christ's glory then was manifested to the disciples three ways. By the rays of light, it was exhibited to their eyes; by the voice, it was declared to their ears, and by the Spirit, to their souls. The last was the most convincing and certain evidence to them of Christ's divinity.

This glory of Christ that the apostles then saw, both the outward glory and that spiritual glory that the outward glory had a semblance of, did most remarkably appear to be such as exceedingly became the only begotten, dearly beloved, and infinitely lovely Son of God. Therefore the apostle John, who was another eyewitness of it, speaking probably with special reference to this, John 1:14, [says], "We beheld his glory, the glory as of the only begotten of the Father, full of grace and truth." It exhibited not only the divine greatness in the majesty, of which the apostle Peter in this place especially speaks, but the divine grace and love in the sweetness of it.1

  1. Thomas Boston Human Nature in its Fourfold State (Falkirk: Printed by Patrick Mair, 1787) p. 23 2. Jonathan Edwards Notes on Scripture (New Haven: Yale University Press) p. 214