If the God is truly Triune, then why did none of the Old Testament prophets ever mention it?
The person asking this question should first of all carefully read the answers we have already given to questions 1, 11 and 23 and in particular the first paragraph of the answer to question 23.
The question appears to assume that the prophets of the Old Covenant explicitly spoke about the central theological doctrines of Christians in the New Covenant, or that they should have spoken about it, if doctrines such as the Trinity, the Incarnation of God’s Word in Jesus Christ and the atoning power of Jesus’ death were true in any way. The prophets in the Bible were, however, no forecasters of future events and doctrines! They were asked and enabled by God to discern His acting and speaking in history, to recognise it and to interpret it.
The God of History
The God of biblical faith is the God of History. The living God who transcends time and space can be experienced in specific, concrete situations, in specific places of prayer. He enters directly into our space and time-bound historical existence, and the manner of His presence is a noticeable, surprisingly new kind of awareness: He comes in a dream to warn (Gen. 20:3; 31:43), He comes enveloped in a cloud to give Moses prestige and standing before the people (Ex 19:9); He comes to the altar to bless (Ex 20:24). He is a God who in a sense can be ‘experienced’. It is an experience that was brought into and from the history of Israel, and an experience that affected individuals. [...] In the drama of these overwhelming historical experiences, which in turn cause deep consternation as well as jubilant hope in the light of God’s love, but also the shameful shock of His punishing harshness and the painful battle for His incomprehensible justice, one thing will become an act of faith among the people: The wholly Other is loving and close to humanity. He remains the mysterious Incomprehensible one, and yet his living care proves Him to be the God for His family, initially for Israel but beyond that also as God for all of humanity and of the world. His care is so much part of Him that it is the essence of His name, His being: I am here for you (cf. Ex 3:13) (Theodor Schneider, “Was wir glauben. Eine Auslegung des Apostolischen Glaubensbekenntnisses”. Düsseldorf: Patmos, 1988, p. 105.)
The central self-proclamation [in the history of the old Covenant and therefore in the Holy Scriptures of this Covenant] can be summarised as an exciting dialectic task: the only, world-transcending, world-independent God who cannot be grasped, who in His absolute uniqueness is powerful and who exists in all eternity, has from His own free will made Himself to be a God for the world and for humanity. The world-transcending God transcends Himself in His personal freedom towards the world and towards humanity and in this free focus reveals His true nature. (ibid.)
The name of the God revealed in Ex. 3:14 is Yahweh contains a short Hebrew sentence: ehyeh asher ehyeh. It is a play on words using the Hebrew verb hayah. It is a short sentence describing the nature of God, a kind of key word for God: He is one who approaches His people, liberating them, He is the open, liberating future, the one approaching us.
This unusual and surprising perceivableness of God, the coming of God is linked with the hoped-for and expected goal of our path, the end of history (cf. Ps 50:2-6). Like the morning star, like the sun God arises every over the community of His faithful people. Then it will finally and forever be daylight for all of creation:
Let the floods clap their hands;
let the hills sing together for joy
at the presence of the Lord, for he is coming
to judge the earth. He will judge the world with righteousness (Ps. 98:8-9)
The disciples recognise in Jesus God’s Word made flesh.
The first believers discover in the death of Jesus, in his resurrection and in the sending of the Holy Spirit the one and only God of Abraham, Isaac and Jacob in a very special way as the father of our Lord Jesus Christ; that is the God to whom Jesus prays in endless trust and whom he tenderly calls ‘Abba’, dearest father, for whom he wants to create space in all of Israel so that his plan for salvation can become true on earth as it is in heaven. Jesus proclaims the approaching Kingdom of the Father and allows it to begin, to break through because of his own actions; in His name and with His authority Jesus forgives sins, heals the sick, restores the dead to life, conquers demonic powers. He represents the Fathers endless compassion and grace, in particularly in his giving himself upon the cross for your and for humanity. He, the elevated and coming Son of Man has been given the right to judge humanity.
Jesus relationship with this Father-God is always very clear: He stands in face of him, he always points to him, he is different from him, because he is a mortal human being just like we are; he is the Son of Man. And yet, because of his life, death and resurrection the early Church sees in him the image of the invisible God (Col. 1:15), the promised Emmanuel, the ‘God-is-with-us’. (Mt 1:23) His closeness to the Father, his trusting and loving relationship with Him and therefore his inner unity with Him were experienced as so close, so intimate and unique that believers realised more and more that despite all differences between Father-God and Jesus, they are also inseparably linked one with the other. Jesus is as the Son simply inseparable from his Father and has always been so from before time and creation, as the everlasting wisdom of God in which God designed creation from within eternity, and that he is the eternal word (logos) of God with which God called creation into being and existence. The wisdom and the word of God are terms already contributed by wisdom theology of the Old Testament, the logos-theology of the Jewish philosopher Philo of Alexandria (at the time of Jesus) and the Rabbinic Thora-theology, and which the early Christians applied to Jesus. For in him Gods wisdom became embodied in time (1 Cor 1:24-31); in him the logos, the Word of God became flesh (Jn 1:14); in him the love of God appeared in human form (Tit 3:4), in him God discovered Himself just as He is and revealed and gave Himself to us.
Using the language of Greek philosophy the great Councils of the first six centuries expressed the experience of early Christians like this: God from God, Light from Light, true God from true God, of one being with the Father (Nicene Creed). While the thinking here moves to the philosophical level of Greek thought, the same applies as to the biblical, more figurative language. These are attempts to think three facts at the same time and to merge them in faith:
(1) The monotheistic God: There is only one God;
(2) The full humanity of Jesus: In all things like unto us, without sin (Council of Chalcedon);
(3) the maximum of conceivable unity between God and Jesus: They are different from each other but are in a unique and inseparable relationship.
Today these are still the three criteria or principles on which all Christian understanding of Jesus must be based, regardless of which images or words each era and each culture may use to express them. The Christological and Trinitarian dogmas of the first Councils are the cross-cultural points of reference that must never be ignored. (Medard Kehl, Phil.-Theol. Hochschule Sankt Georgen. „An den dreieinen Gott glauben. Warum dies kein entbehrliches theologisches Glasperlenspiel in der Begegnung mit dem Islam ist“ (=To believe in the triune God. Why this is not a dispensible glass beads game in the encounter with Islam) [unpublished script]).
The prophets of the old Covenant did not forecast the development of the triune faith and the doctrine of the Holy Trinity. But they recognised and proclaimed in their lifetimes and eras that Yahweh is the one God of their faith: I am here for you who, because of His nature, will grant His people steadfast forgiveness, mercy and faithfulness. Jesus was introduced into this faith by his parents and teachers and could understand his nature and his task as the incarnated word of God from within this faith. The faith in Jesus as the word of God made flesh then opens the way to the full development of the Christian Trinitarian faith.