There are a number of passages in the Old Testament which are often cited in support of the doctrine of the Trinity by Christians. Well known among them are, for example, the verse in the first chapter of Genesis, when God says “Let us make man in our own image” (Genesis 1:26) and thus man is made in the image of God. However, the use of the plural indicates a plurality in God. Another is the episode with Abraham when God appears to him, coming to him in the appearance of three men (Genesis 18:2).
One of the lesser known, and yet I feel more persuasive passages, comes from Isaiah.
In the year that King Uzziah died, I saw the Lord, high and exalted, seated on a throne; and the train of his robe filled the temple. Above him were seraphim, each with six wings: With two wings they covered their faces, with two they covered their feet, and with two they were flying.And they were calling to one another:
“Holy, holy, holy is the LORD Almighty;
the whole earth is full of his glory.”
Catholics are familiar with this verse, as it is sung and rehearsed every single day in the Mass (at least the Latin Mass). Here is a verse which, in the eyes of the Church Fathers, clearly typified the Trinity. For instance, let me provide a brief survey of some commentaries from the Church Fathers.
What is the meaning of the threefold utterance of the word Holy? If it is repreated three times, why is it one act of praise? is not Holy repeated three times because the Father and the Son and the Holy Spirit are one in holiness?… Furthermore, to show that the trinity is one God, after the threefold Holy is spoken they add the singular, the Lord God of hosts… As for me, I believe as the Seraphim believed, and adore God as did all the principalities and powers of heaven.
~ Ambrose of Milan
When Isaiah also praising the unity of the Trinity, he says that the voices of the seraphim cried out: Holy, holy, holy. However, lest he should appear to sever the unity of the divine nature, he added, Lord God of hosts. He did not say “Lords” or “Gods” but Lord God. In that way he indicated that the one who was three times called Holy was one God.
~St. Pope Gregory the Great
The seraphim begin their doxology with a threefold repetition of Holy, and they conclude with the words Lord of Hosts, thereby affirming that the one divine nature is a Holy Trinity. For when we confess the creed we say that there are three persons, the Father, the Son, and the Holy Spirit. There is no reason to divide these three into different natures simply because each bears a different name. The one divine Godhead is to be understood in three persons.
~Cyril of Alexandria
It is also a part of an extremely early liturgy called the Liturgy of St. James, from the ante-Nicene period of the Church. The liturgy contains the following:
Priest: It is truly meet and right, fitting, and our bounden duty to praise You, to laud You, to bless You, to worship You, to glorify You, to give thanks to You, Maker of all things visible and invisible, the treasury of all good things, the source of life and immortality, the God and Lord of all, whom the heavens and the heaven of heavens and all the powers thereof do praise, the sun and moon and all the choir of the stars, earth, sea, and all that is in them, the assembly of the heavenly Jerusalem, the Church of the firstborn whose names are written in the heavens, the spirits of the righteous and prophets, the souls of the martyrs and apostles, angels, archangels, thrones, dominions, principalities, authorities, dread powers, cherubim with many eyes, and the six-winged seraphim who with two wings cover their cases and with two their feet and with two they fly and cry one to the other with ceaseless voices in endless praise singing the hymn of victory of Your excellent glory, with clear voice singing and shouting, glorifying and crying and saying:
People: Holy, Holy, Holy, Lord of hosts, Heaven and earth are full of your glory. Hosanna in the highest! Blessed is he who comes in the name of the Lord! Hosanna in the highest!
~Liturgy of St. James
Sound familiar? Perhaps we should keep in mind next time we burst into song during the liturgy that we are not only, here, joining in the heavenly choir of angels, but we are specifically reflecting the Trinity as we repeat ‘Holy’ three times every Mass.