Is the Holy Trinity mentioned in the Old Testament also?
Even in the Old Testament there are intimations of plurality within the unity of God: “By the Word of the LORD were the heavens made, and all the host of them by the Breath (Spirit) of his mouth” (Psa 33:6 [32:6 LXX]; see Q149); and the thrice-holy hymn of the seraphim: “Holy, Holy, Holy is the LORD of hosts; the whole earth is full of his glory” (Isa 6:3; see Q337), which the Church sings to the Trinity in the Trisagion and at every Divine Liturgy.
In the account of the vision granted to Abraham at the oak of Mamre, the LORD appears in the form of three men, who are addressed as one Lord and who speak for God and as God (Gen 18:1–33; 19:1, 24; see Q462). Abraham ran to meet three visitors, yet bowed and spoke to One.
This appearance of YHWH to Abraham in the form of three visitors was understood by the Fathers as a prefiguration of the Holy Trinity “the Hospitality of Abraham”, and it is this scene that St Andrei Rublev’s celebrated icon of the Holy Trinity depicts: the three angels seated about the one table, on which stands the cup.
Moreover, the several theophanies (manifestations of God) of the Old Testament have always been understood in the Church as appearances of the Word of God before His Incarnation. As the holy martyr Justin the Philosopher teaches, it is the pre-eternal Word, the Son of God, who appeared to the patriarchs and prophets of old. He is the One who spoke with Abraham, wrestled with Jacob, spoke to Moses from the burning bush as the Angel of the Lord, and revealed God to the righteous of the Old Testament (cf. St Justin Martyr, Dialogue with Trypho, chapters 56–62, 75, 126–129; see Q297, Q319).
In the Gospel of John, the Lord Jesus Christ identifies himself with the “I AM” of the Old Testament, “before Abraham was, I AM” (Joh 8:58; cf. Exo 3:14; Isa 43:10–11; 52:6; see Q028, Q503), and the Evangelist identifies him with the enthroned YHWH of Isaiah’s temple vision. For after recounting the Lord’s signs and the people’s unbelief, John cites Isaiah’s vision, “In the year that King Uzziah died, I saw the Lord sitting on a throne, high and lifted up… and one cried to another: ‘Holy, holy, holy is the LORD of hosts’” (Isa 6:1–10; see Q345, Q383), and concludes: “These things Isaiah said when he saw his glory and spoke of him” (Joh 12:37–41).
God the Father, too, is prefigured in special ways in the Old Testament, as in the title “God Most High” (El-Elyon), whose priest was Melchizedek, the king of Salem who brought out bread and wine (Gen 14:18; see Q043); for the angel announced to the Virgin, “the power of the Most High will overshadow you. Therefore, the holy one who is borne will be called the Son of God” (Luk 1:35; see Q191, Q542); and Jesus “having become a high priest forever after the order of Melchizedek” (Heb 6:19–7:1; see Q008, Q225).
Finally, in the very first verses of Scripture, the work of creation is revealed as the work of the Holy Trinity: “In the beginning, God created the heavens and the earth… and the Spirit of God was hovering over the waters. And God said [the Word of God], ‘Let there be light,’ and there was light” (Gen 1:1–3; see Q098, Q537).
