The mountain, in the bible—as, indeed, among all peoples of antiquity—was the site of the encounter with God.
 
In Exodus 24 we find that after six days (Exodus 24:16) Moses went up; he did not go alone, but took Aaron, Nadab and Abihu with him (Exodus 24:19), and was enveloped in a cloud.
 
It is enough to conclude that, with these Old Testament allusions, the evangelist intends to communicate a message. He intends to present Jesus as the new Moses.
 
Recalling the images of the cloud, the shining face and the mountain from the Old Testament, Matthew says that Peter, James, and John, in a particularly significant moment of their lives, were introduced to the world of God and given a taste of the enlightenment, giving them an insight into the true identity of the master and the destination of his journey.
 
He would not be the glorious Messiah they expected, but a Messiah who, after a severe conflict with the religious powers that be, would be opposed, persecuted and killed. They also realised that their fate would be no different from that of the Master.
 
The topic discussed in the previous chapter (Matthew 16) is: “Who do people say that the Son of Man is?” Peter, had expressed the conviction that he is the long-awaited messiah.
 
The voice from the sky—introduced in the story of the transfiguration—declares the opinion of God, “Jesus is the beloved,” the faithful servant of whom God is well pleased (Isaiah 42:1).
 
This voice had already been heard at baptism. “This is my beloved Son” (Matthew 3:17). Now an exhortation is added: “Listen to him.”
 
Listen to him, even when he seems to propose too demanding a path indicating the narrow and steep way, paradoxical and humanly absurd choices.
 
The image of the three tents indicates, perhaps, the desire of Peter to stop, to perpetuate the joy experienced in a moment of spiritual intimacy with the master.
 
The one who builds a tent wants to fix his abode in one place and not move, at least for some time. But Jesus is always on the move. He goes directly to a des  tination and the disciples must follow him.
 
The true rapport with the Lord does not lead to withdrawal into self, does not close you off in a sterile spiritual intimacy.
 
It is necessary to go out to meet and serve the brothers and sisters, to help those who suffer, to be close to anyone in need of love.
 
God intervenes to correct the false interpretation of Peter: Jesus is not just a great legislator or a mere prophet as was Moses or Elijah; he is the beloved Son of the Father.
 
The three cannot continue to be together any longer. Jesus stands out clearly from the others and is absolutely superior.
 
When the disciples looked, Moses and Elijah were gone, they had already accomplished their mission: they presented the Messiah to the world, the new prophet, the new lawgiver.
 
• Father Fernando Armellini SCJ
Claretian Publications
http://bibleclaret.org/celebrating-the-word-of-god/95-blog/sunday-reflection/919-transfiguration-of-the-lord