Matthew 17:5

A bright cloud overshadowed them - The word “overshadow” here means, rather, to “be diffused” or “spread” over them. It does not mean that it made a shade. A cloud was the symbol of the divine presence. Thus, God went before the Israelites in a cloudy pillar - dark by day and bright by night Exodus 14:19-20; he appeared on Mount Sinai in a cloud bright by fire Exodus 24:15-17; and a cloud, the symbol of the divine presence - called the Shechinah - dwelt continually in the most holy place in the temple, 1 Kings 8:10-11; Ezekiel 1:4; Ezekiel 10:4. When, therefore, the disciples saw this cloud, they were prepared to hear the word of the Lord.

This is my beloved Son - This was the voice of God. This was the second time that, in a remarkable manner, God had declared this. See Matthew 3:17. This was spoken to confirm the disciples; to make known to them that it was their duty to hear Christ rather than any other, and to honor him more than Moses and Elijah; and to strengthen their faith in him when they should go forth to preach the gospel after he was shamefully put to death. After this, it was impossible for them to doubt that he was truly the Son of God. See 2 Peter 1:17-18.