O Rex Gentium, et desideratus earum, lapisque angularis, qui facis utraque unum: veni, et salva hominem, quem de limo formasti.
O King of the Nations, and their desire; the Corner-stone, who makest both one; Come and save mankind, whom thou formedest of clay.
The key text here is Haggai 2:8,
'Et movebo omnes gentes, Et veniet Desideratus cunctis gentibus'
'And I shall shake all nations, and the Desired One will come to all nations.'
Haggai is a prophet writing at the time of of what is called the Restoration, that is, the return of the Jews to the holy land after the exile in Babylon, the rebuilding of the temple and the restoration of public and religious institutions. As Haggai writes, these things do not yet amount to much, but he forsees a time when the glory of the restored temple with exceed that of Solomon's original building. Christians see this prophecy fulfilled in Christ.
The phrase 'Rex Gentium' I have not found exactly, but cf. Psalm 2:6-8,
Ego autem constitutus sum Rex ab eo
Super Sion, montem sanctum eius,
Praedicans praeceptum eius.
Dominus dixit ad me: Filius meus es tu;
Ego hodie genui te.
Postula a me, et dabo tibi gentes haereditatem tuam,
Et possessionem tuam terminos terrae.
'Yet have I set my King:
upon my holy hill of Sion.
I will preach the law, whereof the Lord hath said unto me:
Thou art my Son, this day have I begotten thee.
Desire of me, and I shall give thee the heathen [i.e. nations]
for thine inheritance:
and the utmost parts of the earth for thy possession.'
The corner-stone goes back ultimately to Isaiah 28:16,
Ecce ego mittam in fundamentis Sion lapidem,
Angularem, pretiosum, in fundamento fundatum;
'Behold, I lay in Zion for a foundation a stone, a tried stone, a precious
This is quoted at 1 Peter 2:6. St Paul at Ephesians 2:20 also refers to Christ as 'ipso summo angulari lapide Christo Iesu' - 'Jesus Christ himself being the chief corner-stone.' In context, Paul explores the meaning of this image as referring to the Jews and Gentiles as it were coming to God from two directions, and meeting in Christ, as two walls meet and join in the
corner-stone. 'Who makest both one' refers to Ephesians 2:14, 'qui fecit utraque unum'.
'Quem de limo formasti' derives from Genesis 2:7, 'Formavit igitur Dominus Deus hominem de limo terrae.' Again Jesus is identified with the God of Creation, the God of Genesis.
A number of texts have been combined to produce a coherent theology: Christ is the Lord of all nations, both Jews and Gentiles, as a corner-stone supports both walls; he is the agent through whom both were made, and will lead both to a destiny greater than anything in their previous existence.
In addition tomorrow, December 23rd, the last day of the O Antiphons, traditionally has a special antiphon for the Benedictus at Lauds:
Ecce completa sunt omnia, quae dicta sunt per Angelum de Virgine Maria.
Behold, all things are fulfilled, which were spoken by the Angel to the Virgin Mary.
So we are ready for Christmas...