Luke 9:35

And a voice, &c. This is the voice of the Father from the cloud, as if he should say, “I call him not one of my sons, but my true and natural Son, to the resemblance of whom all others are adopted. (St. Cyril) — Not Elias, not Moses, but he whom you see alone, is my beloved Son. (St. Ambrose) — Therefore, it is added: and when the voice was heard, Jesus was alone, lest any one should imagine these words, This is my beloved Son, were addressed to Moses or Elias.” (Theophylactus)

7. It is here added, concerning the cloud that overshadowed them, that they feared as they entered into the cloud. This cloud was a token of God's more peculiar presence. It was in a cloud that God of old took possession of the tabernacle and temple, and, when the cloud covered the tabernacle, Moses was not able to enter (Exod 40 34, 35), and, when it filled the temple, the priests could not stand to minister by reason of it, 2 Chron 5 14. Such a cloud was this, and then no wonder that the disciples were afraid to enter into it. But never let any be afraid to enter into a cloud with Jesus Christ; for he will be sure to bring them safely through it.

Verses 28-36 Christ's transfiguration was a specimen of that glory in which he will come to judge the world; and was an encouragement to his disciples to suffer for him. Prayer is a transfiguring, transforming duty, which makes the face to shine. Our Lord Jesus, even in his transfiguration, was willing to speak concerning his death and sufferings. In our greatest glories on earth, let us remember that in this world we have no continuing city. What need we have to pray to God for quickening grace, to make us lively! Yet that the disciples might be witnesses of this sign from heaven, after awhile they became awake, so that they were able to give a full account of what passed. But those know not what they say, that talk of making tabernacles on earth for glorified saints in heaven.