Is the Trinity in the Old Testament?

We should answer plainly: no, the Old Testament does not teach the developed doctrine of the Trinity. Ancient Israel was fiercely monotheistic - "Hear, O Israel: the LORD our God, the LORD is one" (Deuteronomy 6:4) - and no prophet set out three persons in one essence. That doctrine was articulated at Nicaea and Constantinople, on the basis of the fuller New Testament revelation. Nor should anyone lean on old overreaches, like reading plurality into the Hebrew word for "one."

The narrower, defensible claim is that the Old Testament presents the one God as richer and stranger than a bare mathematical unit - through progressive revelation, shadows cast by something real. There is the plural "Let us make man" (Genesis 1:26); the Angel of the LORD who speaks as the LORD yet is sent by him; a LORD who rains fire "from the LORD out of heaven" (Genesis 19:24); God's Word, Wisdom, and Spirit; and Daniel 7's "son of man" enthroned beside the Ancient of Days.

Each thread, alone, has an innocent reading - the plural may address the heavenly council; the Angel may be an envoy. Those concessions are fair. But the force is convergence: independent devices, across centuries, keep throwing off the same silhouette - one who is of God, with God, and acting as God, distinct yet never a second god. Tellingly, ancient Jews themselves read a "second power in heaven" from these texts, which the rabbis had to argue against. The Old Testament does not by itself reach Nicaea; the New Testament supplies that. What it shows is that the Triune God is exactly the kind of God Israel's Scriptures were already describing.