11 Thus spoke the Lord (it is scarcely sufficient to say), as one dying to dying men; for “to the Lord also belong the issues from death,” as says the psalm. Seeing, then, He was both speaking to those destined to die, and speaking as one appointed to death Himself, what mean His words, “He who keeps my saying shall never see death;” save that the Lord saw another death, from which He had come to deliver us— the second death, death eternal, the death of hell, the death of damnation with the devil and his angels? This is real death; for that other is only a removal. What is that other death? The leaving of the body— the laying down of a heavy burden; provided another burden be not carried away, to drag the man headlong to hell. Of that real death then did the Lord say, “He who keeps my saying shall never see death.”
Source: Tractates on the Gospel of John (New Advent)