Today is the Feast of the Transfiguration. Falling in the middle of the summer months and in close proximity to the Solemnity of the Assumption of the BVM (15th August), it seems to be the forgotten feast day of the liturgical year. It marks the day when Jesus ascended a mountain (traditionally considered to be Mount Tabor) with Peter, James, and John. On the mountaintop, Jesus is transfigured before their eyes with his face and clothes becoming unbearably bright. Both Moses and Elijah at Jesus side and begin conversing with him. Finally, a bright cloud appears and a loud voice declares out of it that Jesus is his beloved Son. On hearing the voice, the three disciples fall to the ground in fear but Jesus reaches out to touch them and reassu. re them. As soon as they look up, Elijah, Moses, and the cloud are gone. They descend from the mountain with Jesus instructing them not to tell anyone about what they had seen until after his Resurrection. This account is found in all three of the synoptic gospels (cf. Mt. 17.1-8, Mk. 9.2-8, Lk. 9.28-36) and is possibly alluded to in John's Gospel (cf. Jn. 1.14).
Thomas Aquinas devotes an entire question in the third part of his Summa to this incredible event in the life of Christ. He writes that the purpose of the Transfiguration was to give the three disciples an encouraging vision of not only the glory that awaited Jesus after his Passion, Death, and Resurrection, but also of the glory that awaited them (and, by extension, us).
We read in the accounts of the Transfiguration that God the Father speaks out of the bright cloud in order to proclaim Jesus' divine sonship. This is, of course, the second time that the Father has done this--the Father spoke out of the cloud at Christ's Baptism (cf. Mt. 3.13-17, Mk. 1.9-11, Lk. 3.21-23). As such, these two events in Jesus' life are closely connected. Jesus came to give Grace by his actions, and to promise Glory by his words. As Aquinas writes, Jesus, at his Baptism, demonstrates to us that first step to receiving Grace--namely, receiving baptism. In turn, the Transfiguration functions as the foreshadowing of the Glory that comes from the triumph of Grace. Aquinas concludes that the Father proclaims Jesus' divine sonship at both the Baptism and Transfiguration in order to emphasize the availability of Grace and Glory to humankind whilst keeping us mindful of the fact that it is only available through Christ, the True Incarnate God.