What are the positions of the Old Testament and the New Testament on the doctrines of the oneness of God and the three Hypostases?

Both the Old and New Testaments affirm the doctrine of One God. But one purpose of the Old Testament was to prevent the faithful from worshipping idols — inanimate gods — and from practicing the abominations of those nations associated with pagan worship (2 Kings 21:2). The New Testament, however, testifies to the one God from a more positive perspective, for it not only proclaims the oneness of God, but also deepens our faith in the

One God by highlighting the Trinity. This does not actually contradict divine oneness but supports it by revealing some of the mysteries of the One God and His relationship with humanity. Without Trinitarian faith, divine oneness would remain obscure. Trinitarian faith is a fundamental path to our salvation, touching our practical lives, our relationship with God, and our eternal future. Such faith causes no dissonance in a mind that is spiritual, but rather fulfills it and exalts it to meditate on divine mysteries with joy.

St. Gregory of Nazianzus³⁶⁰ understands the Old Testament as proclaiming the Father openly, but the Son in a more obscure manner. Then, the New Testament proclaims the Son clearly, and suggests the divinity of the Holy Spirit. Thus, the Trinity was revealed gradually, lest people become like those who are weighed down by eating more food than they can stomach, or like those who gaze upon the sun with eyes too weak to bear its light; lest they therefore lose even that which was within their grasp. But as David says, gradually they ascend, progress, and grow from glory to glory (Ps. 84:7; 2 Cor. 3:18), as the light of the Trinity shines on those who are enlightened.