To introduce us to God
In the Old Testament, God reveals Himself to us. He shows us that He is "I AM WHO I AM" - the Creator, the Living God, the only Savior. God also shows us what He is like - a gracious and just God who is holy and enthroned in heaven. And after revealing Himself, God also shows us how we are to approach Him - in reverent fear, love, and obedience.
To introduce us to ourselves
In the Old Testament, God shows us how He created mankind, what He designed for mankind, and how mankind has all turned away from Him. God shows us that the problem is not so much "out there" as it is "in here" - in our hearts. He gives us the Law as a mirror by which we might examine our hearts and see ourselves in the light of truth. His Law reveals our need for reconciliation, healing, and a new heart. We need a Mediator who will bring us to God and a Savior who will make us whole.
To introduce us to the Savior
In the Old Testament, God gives many illustrations, types, and predictions about how He would reconcile us to Himself and heal us. God introduces to us the idea of substitutionary atonement in the Law and throughout the Law, Prophets, and Writings, tells us how we can recognize the Savior when he comes.
To encourage us
In the Old Testament, God gives us precious wisdom, beautiful words, and awesome examples to encourage us in our walk with Him. The Christian life wasn't designed to be walked alone - and so God gives us living brothers and sisters to help us and the record of past brothers and sisters to refresh our souls through this life of faith.
Praise God for the gift of the Old Testament!