Helpful words on the nature of progressive revelation from D.A. Carson talking about his book–Commentary on the New Testament Use of the Old Testament (Baker, 2008, 1156 pgs)–to Collin Hansen in a CT interview (2/8/08):
“…Sometimes Christians understand progressive revelation in a fairly mechanistic or linear fashion: More truth simply gets added to the pile, to make a bigger pile of truth. But this “mystery/revelation” tension shows that often something is actually there in the Old Testament text (according to Jesus and his apostles) that was not seen until the coming of Jesus made it clear. The most obvious example is the fact that interpreters of Scripture before the coming of Jesus did not happily put together the Old Testament promises of a Davidic king with Old Testament suffering-servant passages to anticipate a king who suffers, a king who would reign from a cross.”