Abraham showed His willingness to sacrifice His own son, and then God intervened at the last second, providing a ram. Once Abraham sacrificed the ram, God declared that he would have numerous descendants who would be a blessing to the earth.
This story is an archetype of Jesus Christ and His sacrifice for us upon the Cross. Lest His death be seen as a morbid dead-end, Christ gave three of the Apostles, and ultimately us, a glimpse of the fullness of His glory. He connected it with the Law of Moses and the words of the prophets that point to Him as the fulfillment of God's plan, since Moses and Elijah appeared with Jesus. And lest we want to hold on to this vision and avoid the suffering of the Way of the Cross, after St. Peter spoke, God shined a bright light and commanded the apostles to listen to Jesus, the beloved Son.
And then they were commanded to wait before telling this vision, because God had more work to do in them to reveal the full understanding of His plan for salvation.
We are encouraged by these glorious visions because we know that God has done so much to save us, even offering His own Son, and demonstrates His power over death through the Resurrection, to which the Transfiguration points.
Truly we can count on God through these glimpses of the fullness of His presence.
As we seek to fully the Way of the Cross, we, too, can join the number of Abraham's descendants who are a blessing to the world, as God's glorious vision works in us.