This mystery is one of my favorite Gospel mysteries of the Rosary. Peter, James and John, who would be present in the garden at Gethsemane when Jesus was taken into custody to face His eventual death, are privileged to see the Lord in His glorified body and to hear the voice of the Father. What the Father says is essentially what the Mother said at Cana - listen to Him, or, do whatever He tells you. The disciples will soon have their faith shaken to the core when the Lord undergoes His unspeakable Passion and Death on the cross. It has been preached that Jesus allowed the three apostles to view His Transfiguration because they would soon see just the opposite. Knowing the human mind, did Our Lord want them to have another image to refer to rather than one of Him in chains or worse, hanging from the cross? And perhaps knowing that they might ask themselves, as so many do in our world today, what kind of father would ask his son to die,would hearing the Father's words affirm for them that Jesus was carrying out His Father's will? Though they would not understand all that was to happen, they were fortified to accept it.
"Lord, it is good that we are here." Yes, perhaps not only because what they had seen on Mt. Tabor would strengthen them for the crucifixion, but it would also prepare them for the Resurrection.
In John Chapter 20, Jesus tells the doubting apostle Thomas "You believe because you have seen, blessed are those who have not seen me but believe." It isn't easy, and no one understood that better than Our Lord. The Transfiguration is affirmation of that.