It was William Dyrness who opened my eyes – with the beauty and power of a well-chosen metaphor – to how the two testaments relate to each other.
One way of clarifying this relationship between the testaments is to liken the Bible to a symphony. All the basic themes of the symphony are presented in the OT and can seen and enjoyed on their own terms. All the reality of God’s self-revelation in creation and redemption comes to expression in these themes. There is a real movement of God toward humankind and a real fellowship between them – not just the promise of such movement and fellowship.
The NT then takes these themes, develops them and, while adding melody lines of its own, transposes the whole into a higher key, weaving everything together in a rich and beautiful way. What was a simple melody line in the OT – say, for example, the discouragement and provision of the wilderness wanderings – is picked up in another setting and made to enhance the NT revelation – as in Paul’s warnings and encouragement to the Corinthians Church (1 Cor. 10).
If we do not listen carefully to the OT we may miss some of the most moving melodies of the NT. So rather than seeing the OT as temporary or partial – something to be outgrown and discarded – we see its incompleteness more as cords calling for resolution, or, to change the metaphor, as plots calling for denouement. What the NT gives us then is does not really leave the OT behind so much as bring out its deepest reality. One has the feeling that in going ever more deeply into the reality of the OT one comes to the truth of the NT. The NT and the OT call for each other for their full self-expression.
Themes in Old Testament Theology by William Dyrness (IVP, 1977); pp.18-19