Theophylact of Ohrid

That is, the glory in which the righteous shall be. Now He said this of His transfiguration, which was the type of the glory to come; as if He said, There are some standing here, Peter, James, and John, who shall not reach death before they have seen at the time of My transfiguration what will be the glory of those who confess Me.

Catena Aurea by Aquinas ↗

Ambrose of Milan

Thus, if we wish not to fear death, let us stand where Christ is, so that he may say of us too, "There are some standing here that shall not taste death." It is not enough to stand unless the standing is where Christ is, for only those who can stand with Christ cannot taste death. It is therein lawful through the quality of the very word to ponder that those who are seen to have deserved the fellowship with Christ will not have even the perception of death. …

Commentary on Luke ↗

Gregory the Dialogist

He continues: "But I say to you truly, there are some standing here who will not taste death until they see the kingdom of God." The kingdom of God, dearest brothers, is not always called the coming kingdom in sacred Scripture, but sometimes the present Church is so called. Hence it is written: "The Son of man will send his angels, and they will gather out of his kingdom all causes of scandal." In that kingdom, indeed, there will be no scandals, where certainly the reprobate are not admitted. …

Forty Gospel Homilies, Homily 32 ↗

Bede

But I tell you truly, there are some standing here who will not taste death before they see the kingdom of God. The kingdom of God in this place is called the present Church; and because some of the disciples were to live in the body to such an extent that they would see the Church of God constructed and raised against the glory of this world, it is now said with a comforting promise: There are some standing here who will not taste death before they see the kingdom of God. …

On the Gospel of Luke ↗