At the baptism of our Lord, where the mystery of the first regeneration was declared, as at the Transfiguration which manifested the second, the whole Trinity appeared: the Father in the voce, the Son in His Humanity, the Holy Ghost under the form, first of a dove, and afterwards of a bright cloud. For if in baptism this Holy Spirit confers innocence symbolized by the simplicity of the dove, in the Resurrection he will give to the elect the brightness of glory and the refreshment after suffering which are signified by the luminous cloud.
But without waiting for the day when our Saviour will renew our very bodies conformable to the bright glory of His own divine Body, the mystery of the Transfiguration is wrought in our souls already here on earth. It is of the present life that St. Paul says and the Church sings today: God, who commanded the light to shine out of darkness, hath shined in our hearts, to give the light of the knowledge of the glory of God, in the face of Christ Jesus [II Cor. 4:6]. Tabor, holy and divine mountain rivalling heaven, how can we help saying with Peter: “It is good for us to dwell on thy summit!”
Commentary from The Liturgical Year by Dom Prosper Guéranger (1805-1875)