Albert Barnes Commentary Luke 9:31

"who appeared in glory, and spake of his decease which he was about to accomplish at Jerusalem." — Luke 9:31 (ASV)

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 Barnes on 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 simply going forth from a land of captivity and servitude 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.