In St. Francis of Assisi, Chesterton spends a significant part of Chapter Two explaining why we must begin telling the life of a person by explaining origins and development the historical forces that are in play when the person arrives on the scene. He is preparing for his main thesis, that St. Francis came at the end of a long penitential season for Europe, by which Christendom and its approach to nature was purged of the corrupting influence of the perverted gods of Paganism. "Until we understand, not necessarily in detail, but in their big bulk and proportion that pagan progress and that Christian reaction, we cannot really understand the point of history at which Saint Francis appears or what his great popular mission was all about."
It has always struck me that the same can be said about the New Testament. How can be understand Jesus without fairly well-developed knowledge of the Old Testament? (Or the Greco-Roman context, for that matter--another argument for a classical education). I find it very difficult to read a passage from the New Testament without having access to the Old Testament to check out allusions. I carry a little New Testament in my back pack. When I want to meditate on the Gospels, I feel like I am holding an incomplete book, because I can't access the OT passages that are in the marginal notes.
As a random example, how much more rich is our reading of the story of Elizabeth and Zechariah in Luke 1 if we read in in light of the many Old Testament who had singular children through an act of God? The NAB footnote lists them as: Sarah (Genesis 15:3; 16:1); Rebekah (Genesis 25:21); Rachel (Genesis 29:31; 30:1); the mother of Samson and wife of Manoah (Judges 13:2-3); Hannah (1 Sam 1:2).
Or how much better can we understand John 7-8 if we understand the OT background of the Feast of Tabernacles? According to the NAB, "(Sukkoth, Ingathering: Exodus 23:16; Tents, Booths: Deut 16:13-16), with its symbols of booths (originally built to shelter harvesters), rain (water from Siloam poured on the temple altar), and lights (illumination of the four torches in the Court of the Women)."
Matthew even begins with a shorthand review of the OT background of Jesus. If we don't know our OT, we can't get the allusions in the genealogy.
Ignorance of the OT is ignorance of Christ.