I absolutely love this quotation by the great Princeton theologian Benjamin B. Warfield on the Trinity in the Old Testament. Although there isn’t explicit revelation about the Trinity in the Old Testament, we can see this foundational truth clearly by the light of the New Testament:
“This is not an illegitimate reading of New Testament ideas back into the text of the Old Testament; it is only reading the text of the Old Testament under the illumination of the New Testament revelation. The Old Testament may be likened to a chamber richly furnished but dimly lighted; the introduction of light brings into it nothing which was not in it before; but it brings out into clearer view much of what is in it but was only dimly or even not at all perceived before. The mystery of the Trinity is not revealed in the Old Testament; but the mystery of the Trinity underlies the Old Testament revelation, and here and there almost comes into view. Thus the Old Testament revelation of God is not corrected by the fuller revelation which follows it, but only perfected, extended and enlarged.”
Benjamin B. Warfield, The Works of Benjamin B. Warfield Vol. 2 (Oxford: Oxford University Press, 1932; repr. Grand Rapids: Baker, 2003), 141-142.