To answer your good question, let’s read Matthew 17:1-6, “And after six days Jesus taketh Peter, James, and John his brother, and bringeth them up into an high mountain apart, And was transfigured before them: and his face did shine as the sun, and his raiment was white as the light. And, behold, there appeared unto them Moses and Elias talking with him. Then answered Peter, and said unto Jesus, Lord, it is good for us to be here: if thou wilt, let us make here three tabernacles; one for thee, and one for Moses, and one for Elias. While he yet spake, behold, a bright cloud overshadowed them: and behold a voice out of the cloud, which said, This is my beloved Son, in whom I am well pleased; hear ye him. And when the disciples heard it, they fell on their face, and were sore afraid.”

In the previous chapter, the Lord spoke of His glorious return to the earth.  We read in Matthew 16:27-28, “For the Son of man shall come in the glory of his Father with his angels; and then he shall reward every man according to his works. Verily I say unto you, There be some standing here, which shall not taste of death, till they see the Son of man coming in his kingdom.”  The transfiguration, which took place six days later, was a witness to the yet future coming of the Son of Man in the glory of the Father to receive the Kingdom. Three of His disciples saw this incredible event.  When coming down from the mount, the Lord instructed Peter, James, and John saying, “Tell the vision to no man, until the Son of man be risen again from the dead.” (Verse 9). 

The transfiguration scene is typical of the second, visible and glorious coming of our Lord and so after His resurrection Peter wrote about it.  We read in 2 Peter 1:16-18, “For we have not followed cunningly devised fables, when we made known unto you the power and coming of our Lord Jesus Christ, but were eyewitnesses of his majesty. For he received from God the Father honour and glory, WHEN THERE CAME SUCH A VOICE TO HIM from the excellent glory, This is my beloved Son, in whom I am well pleased. And this voice which came from heaven we heard, when we were with him in the holy mount.” 

This shows us clearly that in Matthew 17:5, the Father was speaking concerning His Son when He said, “This is my beloved Son, in whom I am well pleased; hear ye him.”  This is not the only time that God spoke from Heaven and declared His pleasure with His son, the Lord Jesus.  On the occasion of the Lord’s baptism by John the Baptist we read in Matthew 3:16-17, “And Jesus, when he was baptized, went up straightway out of the water: and, lo, the heavens were opened unto him, and he saw the Spirit of God descending like a dove, and lighting upon him: And lo a voice from heaven, saying, This is my beloved Son, in whom I am well pleased.” 

What a wonderful testimony to the person of Jesus Christ.  God declared openly and publicly that Jesus Christ is His Son.  Not only is Jesus Christ the Son of God, He is also God, the Son.  God Himself again declares of His Son in Hebrews 1:8, “But unto the Son he saith, Thy throne, O God, is for ever and ever: a sceptre of righteousness is the sceptre of thy kingdom.”  God asserts that His Son (Jesus) is God!  What a remarkable statement regarding the person of the Lord Jesus Christ, so that we have no doubt as to who He truly is!  (404.4)