On August 6 we celebrate the Feast of the Holy Transfiguration of Jesus. It is one of the 12 major feasts in the Byzantine liturgical calendar. "The Fathers refer to Christ’s transfiguration as to His 'second epiphany' or the second manifestation of His divinity." Feast Of The Holy Transfiguration And Blessing Of The Fruits According To Byzantine Rite Tradition - Archeparchy of PittsburghArcheparchy of Pittsburgh (archpitt.org)
Matthew describes the transfiguration in Chapter 4 of his Gospel (starting with verse 17):
17 Now after six days Jesus took Peter, James, and John his brother, led them up on a high mountain by themselves; 2 and He was transfigured before them. His face shone like the sun, and His clothes became as white as the light. 3 And behold, Moses and Elijah appeared to them, talking with Him. 4 Then Peter answered and said to Jesus, “Lord, it is good for us to be here; if You wish, let us make here three tabernacles: one for You, one for Moses, and one for Elijah.”
5 While he was still speaking, behold, a bright cloud overshadowed them; and suddenly a voice came out of the cloud, saying, “This is My beloved Son, in whom I am well pleased. Hear Him!” 6 And when the disciples heard it, they fell on their faces and were greatly afraid. 7 But Jesus came and touched them and said, “Arise, and do not be afraid.” 8 When they had lifted up their eyes, they saw no one but Jesus only.
9 Now as they came down from the mountain, Jesus commanded them, saying, “Tell the vision to no one until the Son of Man is risen from the dead.”
The Apostles received quite a gift in the preview of the Resurrection through the transforming of Jesus, as a mortal, to Jesus, the immortal. The importance of this vision is crucial to the understanding of the dual natures of Christ. The incarnation of our Lord was God becoming man; the transfiguration is showing the future of man, becoming God-like, or Theosis. "Theosis, the Greek term for deification, means that, because God has become one of us, we can become like Him. He is the only truly Holy One, yet we can become holy by sharing in His life. Because of the incarnation, the impossible has become possible: we can become perfect as our heavenly Father is perfect. . . . With the Incarnation, this likeness was restored to mankind in the person of the Lord Jesus. He was a 'new Adam,' the man that God intended. Christ communicated a share in this restored likeness to others after His death and resurrection. By being united to Him in baptism, we could become by God’s grace 'partakers of the divine nature' (2 Peter 1:4). We no longer relate to God simple as creature to Creator, but as sharers in His own life." » Incarnate for Our Sake (melkite.org) Thus, seeing the vision of Jesus transfigured into God in front of their eyes, the Apostles receive confirmation and a preview of the eternal life prepared for us.