St. Nicholas Cabasilas ca. 1323-1391
First of all every holy rite begins with the doxology: “Blessed is the Kingdom of the Father and of the Son and of the Holy Spirit.” Intercourse with God consists of thanksgiving, doxology, confession, and petition. The first of these is doxology, because when grateful servants approach their Master it is fitting that they should begin not by pushing their own affairs into the foreground, but should concentrate on those their Master…Thus doxology has first place in any intercourse with God, and it is for this reason that the priest glorifies God before any prayer and sacred homily. But why does he glorify the threefold nature of God and not His unity? For he does not say “Blessed be God” or “Blessed be the Kingdom”, but distinguishes the Persons. “Blessed be the Kingdom of the Father and of the Son and the Holy Spirit.” It is because it was through the Incarnation of the Lord that mankind first learned that God was three Persons, and the mystery which is being performed is centered in the Incarnation of the Lord, so that from the very beginning the Trinity must shine forth and be proclaimed. (A Commentary on the Divine Liturgy)
