Monergism

It would seem clear that we must recognize in the Old Testament doctrine of the relation of God to His revelation by the creative Word and the Spirit, at least the germ of the distinctions in the Godhead afterward fully made known in the Christian revelation. And we can scarcely stop there. After all is said, in the light of the later revelation, the Trinitarian interpretation remains the most natural one of the phenomena which the older writers frankly interpreted as intimations of the Trinity; especially of those connected with the descriptions of the Angel of Jehovah no doubt, but also even of such a form of expression as meets us in the "Let us make man in our image" of Gen. 1:26, for surely verse 27: "And God created man in his own image," does not encourage us to take the preceding verse as announcing that man was to be created in the image of the angels. 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.
B.B. Warfield from The Biblical Doctrine of the Trinity