It's quite quotable, which is why the readers of this blog may have run across it before. But for those who haven't, I think this assessment of the light the New Testament sheds on the Old is helpful:
"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 that follows it, but only perfected, extended and enlarged."—Benjamin Breckinridge Warfied, Biblical Doctrines (New York: Oxford University Press, 1932; reprint, Grand Rapids: Baker, 2003), 141-42. You can also purchase the Banner of Truth reprint.