If you knew that the Old Testament had one overarching purpose, would it change the way you handled it?
In Secret Church 1: Survey of the Old Testament, David Platt says that it was written to reveal how God redeems His people for His kingdom. Knowing that the Old Testament has one unifying storyline affects at least three areas of our lives.
It will affect the way we understand. It is going to keep us from fragmenting it. This is what we do: we take the Old Testament, and we fragment it into all kinds of different pieces. Then we can't put it together, get frustrated, and we move on to the New Testament. That hampers our ability to understand what God desires to teach us. We need to know the story.
The beautiful thing is that as we look at God's work in history, we realize that the God who was working in their life is also the God that is working in our lives. We also realize that, if there is a story that is begun in the Old Testament, then that story is still continuing today. We are a part of that story. We want to apply it to our lives. We want to know it. We want to be able to apply what God is doing in all of history. If God is doing something in all of history, don't you want to know so you can apply it to your life?
There are all kinds of worldviews, ideologies, and world religions in our culture today that are teaching things that are false and against the story of God. If we know the story and we proclaim it, then we can show its beauty and its grace and its truth amidst all the diverse and competing worldviews that are present today.
For more on this, check out Secret Church 1: Survey of the Old Testament.