Writing in 1936, J. Gresham Machen wrote:

The writers of the Bible did know what they were doing when they wrote. I do not believe that they always knew all that they were doing.

I believe that there are mysterious words of prophecy in the Prophets and the Psalms, for example, which had a far richer and more glorious fulfillment than the inspired writers knew when they wrote.

Yet even in the case of those mysterious words I do not think that the sacred writers were mere automata.

They did not know the full meaning of what they wrote, but they did know part of the meaning, and the full meaning was in no contradiction with the partial meaning but was its glorious unfolding. (Machen, The Christian Faith in the Modern World [reprint, Grand Rapids: Eerdmans, 1947], 55)

Greg Beale, in a forthcoming article for the Westminster Theological Journal on “The Cognitive Peripheral Vision of Biblical Authors”—adapted as an appendix in the new book co-authored with Ben Gladd, Hidden But Now Revealed: A Biblical Theology of Mystery (IVP Academic, 2014)—agrees with Machen and offers an evocative metaphor to describe the relationship:

Machen is referring to meanings of Old Testament authors that lie at the “edges” of the widest part of their cognitive peripheral vision. There is a blurring at these edges, just as there is with the peripheral vision of our literal eyes. Because of this blurring, one can, therefore, say that these authors may not have been very aware at all of these meanings; but God, who inspired them, was explicitly aware, and when this meaning becomes explicit in the New Testament, the “blurred vision” becomes clear and it is truly something that is organically “unfolded” from the Old Testament author’s original meaning. (p. 364)

Here is Professor Beale delivering this lecture for his inauguration as the J. Gresham Machen Chair of New Testament at Westminster Theological Seminary (September 25, 2013):