Also at the Carey there was an allusion to this quotation from Warfield on the Trinity in the Old Testament. I found the reference here.
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 is not corrected by the fuller revelation which follows it, but only perfected, extended, and enlarged.
(B.B. Warfield, “The Doctrine of the Holy Trinity,” quoted by John Frame in The Doctrine of God, 631)