St. Thomas says that to go straight along a road, you must know the end, where you are going: “In order that anyone go straight along a road, he must have some knowledge of the end: thus an archer will not shoot the arrow straight unless he first see the target” (3a. 45, 1). Thomas tells us that it’s especially important to know the end, “when hard and rough is the road, and heavy the going” (3a. 45, 1).
Thomas tells us that after Jesus had foretold his own Passion, He told them that they too must suffer: “Our Lord, after foretelling His Passion to His disciples, exhorted them to follow the path of His sufferings (Mt. 16:21,24)” (3a. 45, 1).
Just as the disciples will share His sufferings, they will share His glory: “Therefore it was fitting that He should show His disciples the glory of His clarity (which is to be transfigured), to which He will configure those who are His: ‘He will change our lowly body to be like His glorious body’ (Phil. 3:21).
St. Thomas asserts that Jesus’ glory could have always shown during His life but it did not because it was necessary for Jesus to live our human life and to suffer: “That the glory of His soul did not overflow into His body from the first moment of Christ’s conception was due to a certain Divine dispensation, that He might fulfil the mysteries of our redemption in a passible body. This did not, however, deprive Christ of His power of outpouring the glory of His soul into His body. And this He did, as to clarity, in His transfiguration” (3a. 45, 2).
Jesus’ glory shown through His humanity, which is a body like ours: “By His transfiguration Christ manifested to His disciples the glory of His body, which belongs to men only and not angels” (3a. 45, 3).
Just as Jesus shares His sufferings with us, He shares His glory, as Thomas tells us: “To this glory He brings those who follow the footsteps of His Passion, according to Acts 14:21: ‘Through many tribulations we must enter into the kingdom of God’” (3a. 45, 1).
The disciples hear the Father’s voice. St. Thomas says that they are hearing the eternal declaration of the Father who utters the only-begotten and co-eternal Word: “The words are to be understood of God’s eternal speaking, by which God the Father uttered the only-begotten and co-eternal Word” (3a. 45, 4). The Father continually declared “You are My Beloved Son” during Jesus’ time on earth.
According to Thomas, we are being transformed into the image of the Son both imperfectly during our lives and perfectly in heaven: “The adoption of the sons of God is through a certain conformity of image to the natural Son of God. Now this takes place in two ways: first, by the grace of the wayfarer, which is imperfect conformity; secondly, by glory, which is perfect conformity” (3a. 45, 4).
The First Letter of John announces: “We are now the sons of God, and it has not yet appeared what we shall be: we know that, when He shall appear, we shall be like to Him, because we shall see Him as He is” (1 Jn. 3:2).
We begin to take the image of the Son in Baptism and then more perfectly in our eventually union with Christ. Thus the Father bodily declared that Jesus was His Son at His Baptism and at the Transfiguration, “to show the different ways in which men can be partakers of the likeness of the eternal Sonship” (3a. 45, 4, ad 1). makes the comparison to us that we are adopted as God’s sons through conformity of image to the natural Son. First by the grace that is given to us in baptism. That the process of our being conformed to the image of Jesus has begun and it will reach its culmination, as the Letter of John says, “We are now sons of God, and what we shall be has not appeared. When He shall appear we will be like Him because we shall see Him as He is.
The Transfiguration is not just a moment of the disciples seeing Jesus clearly but also our own seeing ourselves as we really are.