In glory - Of a glorious appearance. Of an appearance like that which the saints have in heaven.
His decease - literally, his "exit" or "departure." The word translated here "decease" - that is, exit, or "going out" - is elsewhere used to denote death. See 2-Peter 1:15. Death is a departure or going out from this life. In "this" word there may be an allusion to the "departure" of the children of Israel from Egypt. As that was going out from "bondage," pain, and humiliation, so death, to a saint, is but going forth from a land of captivity and thraldom to one of plenty and freedom; to the land of promise, the Canaan in the skies.
He should accomplish - Which was about to take place.

His decease - Την εξοδον αυτον, That going out (or death) of his. That peculiar kind of death - its nature, circumstances, and necessity being considered. Instead of εξοδον, thirteen MSS. have δοξαν, glory. They spoke of that glory of his, which he was about to fill up (πληρουν) at Jerusalem. The Ethiopic unites both readings. The death of Jesus was his glory, because, by it, he gained the victory over sin, death, and hell, and purchased salvation and eternal glory for a lost world.

Who appeared in glory, and spake of his (h) decease which he should accomplish at Jerusalem.
(h) What death he would die in Jerusalem.

Who appeared in glory,.... In a very glorious manner, in most divine and beautiful forms to Christ, and to his disciples:
and spake of his decease, which he should accomplish at Jerusalem; the word, rendered "decease", is "Exodus", the name of the second book of Moses; so called from the departure of the children of Israel out of Egypt, it gives an account of; and which departure is expressed by this word, in Hebrews 11:22 and to which the allusion is here. Death is a departure out of this world, and goes by this name, 2-Peter 1:15 and so here it signifies Christ's death, or exit, which he was to make at Jerusalem; and Moses and Elias talk with him about this; the nature, manner, use, and near approach of it; and to which they might encourage him, as man. The sufferings and death of Christ were what Moses and the prophets had foretold; and these two speak of the same things now; and which must serve to confirm what Christ a few, days ago had showed his disciples, that he must go to Jerusalem, and there suffer and die. Dr. Lightfoot thinks that the ascension of Christ to heaven is contained in the word "Exodus", which was his final departure out of this world, as well as his sufferings and death; and especially if there is any, allusion to the Israelites' departure out of Egypt, which was in victory and triumph; and the rather, because "the time of his receiving up", Luke 9:51 may be thought to refer to this; and so Moses and Elias conversed with him, not only about, his sufferings and death; but his ascension, and of which also the "Exodus", or going of Elias out of this world to heaven, which was by a translation and ascension, was a figure. Some render this word, "Expedition", and think it refers to the whole affair of the redemption of Christ's people by him, from the bondage of sin, Satan, and the law, by his sufferings and death; of which the deliverance of the people of Israel out of Egypt was a lively representation: an expedition which Moses was sent upon, and accomplished: but now the discourse turns upon an expedition of greater importance, which shortly was to be accomplished at Jerusalem, where Christ was to be arraigned, and condemned, and then suffer without the gates of it, in order to obtain eternal redemption for the whole Israel of God.

spake--"were speaking."
of his decease--"departure"; beautiful euphemism (softened term) for death, which Peter, who witnessed the scene, uses to express his own expected death, and the use of which single term seems to have recalled the whole by a sudden rush of recollection, and occasioned that delightful allusion to this scene which we find in 2-Peter 1:15-18.
which he should accomplish--"was to fulfil."
at Jerusalem--Mark the historical character and local features which Christ's death assumed to these glorified men--as important as it is charming--and see on Luke 2:11. What now may be gathered from this statement? (1) That a dying Messiah is the great article of the true Jewish theology. For a long time the Church had fallen clean away from the faith of this article, and even from a preparedness to receive it. But here we have that jewel raked out of the dunghill of Jewish traditions, and by the true representatives of the Church of old made the one subject of talk with Christ Himself. (2) The adoring gratitude of glorified men for His undertaking to accomplish such a decease; their felt dependence upon it for the glory in which they appeared; their profound interest in the progress of it, their humble solaces and encouragements to go through with it; and their sense of its peerless and overwhelming glory. "Go, matchless, adored One, a Lamb to the slaughter! rejected of men, but chosen of God and precious; dishonored, abhorred, and soon to be slain by men, but worshipped by cherubim, ready to be greeted by all heaven. In virtue of that decease we are here; our all is suspended on it and wrapped up in it. Thine every step is watched by us with ineffable interest; and though it were too high an honor to us to be permitted to drop a word of cheer into that precious but now clouded spirit, yet, as the first-fruits of harvest; the very joy set before Him, we cannot choose but tell Him that what is the depth of shame to Him is covered with glory in the eyes of Heaven, that the Cross to Him is the Crown to us, that that 'decease' is all our salvation and all our desire." And who can doubt that such a scene did minister deep cheer to that spirit? It is said they "talked" not to Him, but "with Him"; and if they told Him how glorious His decease was, might He not fitly reply, "I know it, but your voice, as messengers from heaven come down to tell it Me, is music in Mine ears."

In glory - Like Christ with whom they talked.