Εὐλογημένη ἡ Βασιλεία τοῦ Πατρὸς ϗ τοῦ Υἱοῦ ϗ τοῦ Ἁγίου Πνεύματος, νῦν ϗ ἀεὶ ϗ εἰς τοὺς αἰῶνας τῶν αἰώνων.Blessed is the Kingdom of the Father, and of the Son, and of the Holy Ghost, now and ever and unto the ages of the ages. Amen.
[ϗ is the abbreviation for καὶ]
Thus begins the Divine Liturgy according to the Byzantine Rite, as the priest makes the sign of the Cross with the Gospel-book. Perhaps this is based on Tobit xiii, 1b: Εὐλογητὸς ὁ Θεὸς ὁ ζῶν εἰς τοὺς αἰῶνας καὶ ἡ βασιλεία αὐτοῦ (Blessed be God that liveth for ever, and blessed be His Kingdom).
But note that the Kingdom is of the Three Persons of the Trinity. The Trinity is King!
At the Divine Liturgy, the second ecphonesis, and the well-known doxology after the Lord's Prayer, are almost the same in their proclamation: "For Thine is the Kingdom and the power and the glory, of the Father, and of the Son, and of the Holy Ghost, now and ever and unto the ages of the ages. Amen."
We in the West rarely call the Holy Ghost our King - but in the Byzantine Rite, every sacred act begins with a complex of prayers beginning with this invocation of the Holy Ghost, somewhat answering to the Latin Veni Sancte Spiritus: "O heavenly King, Paraclete, Spirit of Truth..."
Of course, in East and West and wherever Christians are found, we all pray in the Lord's Prayer, "Our Father... Thy Kingdom come... on earth as it is in heaven." (Pater noster... adveniat regnum tuum... sicut in cælo et in terra.) God the Father is King we know, for (as at the Mass in the Western recension of the Gloria in excelsis), in the Byzantine Rite, their equivalent Great Doxology - sung at their Lauds and other Hours - includes the words Κύριε βασιλεῦ, ἐπουράνιε Θεέ, Πάτερ Παντοκράτορ ("Lord King, heavenly God, Father Almighty"): which is simply a slightly different division of words to the Latin Rex cælestis, Deus Pater omnipotens ("heavenly King, God the Father Almighty").
And what of the Great Entrance? The sublime Cherubicon is chanted: "...let us receive the King of all, invisibly escorted by the Angelic orders." Whiles the oblations are processed, which shall soon be consecrated, and so are already types of the Body and Blood of Christ, the priest and people cry, "May the Lord our God remember us all in His Kingdom... Lord, remember me in Thy Kingdom" (quoting St Dismas, the Good Thief: most blessed thief, who even stole Paradise!). Here Christ, our King and our God, is confessed, just as He is at the opening of each canonical Hour, in a threefold cry based on Psalm 94:6 (compare its use at Matins in the West):
Come, let us worship and bow down to the King, our God.Come, let us worship and bow down to Christ the King, our God.Come, let us worship and bow down to Christ Himself, the King, our God.
No one can say that the Byzantine Rite omits homage to Christus Rex!
The Three Persons are each King: the Father is King, and the Son is King, and the Holy Ghost is King: but they are not Three Kings, but One King. (Cf. Athanasian Creed.)
The genius of the Latin Liturgy is to be found encapsulated in the conclusion of the standard Roman Collect: Per Dominum nostrum Jesum Christum Filium tuum, qui tecum vivit et regnat in unitate Spiritus Sancti, Deus, per omnia sæcula sæculorum. Addressing God the Father, we pray through His Son, Jesus Christ, our Lord, Who with Him liveth and reigneth - as King - in the unity of the Holy Ghost, endlessly. Again, the Three are One King.
As for the Latin Mass, no less than in the Byzantine Rite, the Niceno-Constantinopolitan Creed solemnly declares that Cujus [Christi] Regni non erit finis - "Of whose [Christ's] Kingdom there shall be no end". St Cyril of Jerusalem - in his mystagogical catechesis to neophytes - energetically declares, "If thou shouldst hear that Christ's Kingdom hath an end, then hate this heresy!" Similarly, by confessing that Christ sits at the right of the Father, in the Gloria in excelsis and Creed, we declare indirectly that Our Lord Jesus Christ shares in the Kingship of His Father.
The Ascension of Our Lord into heaven, and His perpetual Session at the right hand of God, ruling all things in heaven and on earth, is the proximate foundation of the feast of Christ the King (which was only belately inserted into the Liturgy in 1925) - not that Christ, the Incarnate Word, was ever not our Sovereign Lord, but that, having conquered sin, Satan, death and hell by His Resurrection, He returned in triumph to reign in heaven. There, invisibly maybe but very truly, He rules even this world, which is totally under His sway and the direction of His Providence; for the moment, as this the last age rapidly draws to its ineluctable Judgement and end, He permits men still to sin and waste time away; but the sands in the hourglass are running out. God has appointed a day when Christ shall return, "to judge the living and the dead and the world by fire". May He grant us the grace and mercy to stand and be saved on that Day! (The Neo-Gallican Missals and Breviaries were not far wrong when they marked the Octave Day of the Ascension by a special feast in honour of Christ's Return as our Dread Judge.)
How we ought then call on Him Who is the King of Glory! In the Office, the Te Deum includes the stirring words, Tu Rex gloriæ, Christe: Thou art the King of glory, O Christ. And in Lent, instead of Alleluia, is sung Laus tibi, Christe, Rex æterne gloriæ: Praise unto Thee, Christ, King of eternal glory. On Palm Sunday, we sing the hymn with the refrain Rex Christe, Redemptor: King Christ, Redeemer.
Finally, it ought go without saying - as well the early Christians and the pagan Romans knew - that to call Christ (or for that matter the Holy Spirit) "Lord" is to solemnly acknowledge that this Person is King and God, ranking with the Eternal Father.