One of the challenges in teaching biblical theology or redemptive history is to find ways of explaining the continuity and change between the administrations of the covenant of grace (and of course in framing the question like that I have already ruled out many answers!). What did the Old Testament believer have and what was still to come? Warfield has perhaps the most famous illustration (on the Trinity in the Old Testament: the room was richly furnished but dimly lit). In A History of the Work of Redemption Edwards has a similar, light-related picture: ‘The church indeed had the light of the sun then but it was but as reflected from the moon and the stars’. I like this because it suggests the idea of the same grace being administered but by different means, i.e. via the shadows and types. Do you have an illustration at the ready? And, more importantly, what understanding of change and continuity stands behind it?
Putting it crudely, the great puzzle is how we are to account for both the positive and the negative ways in which the Mosaic era is presented: Scripture says that the law gives life and it kills. It was an era in which the gracious and compassionate God redeemed, and yet it was a ministry of condemnation. As I have wrestled with this the most useful insight I have found is that when Paul speaks negatively about the law he is usually speaking about what it had become for Israel in her history. Galatians 4 is a good example of this when he speaks of the ‘now Jerusalem’ (v. 25). This means that he has in view her corporate unbelief and rejection of the covenant. Individuals like Abraham or Joshua were genuine believers saved like believers under the new covenant, but the nation as a whole fell into unbelief, hence the exile. Received thus, the law kills. But the law in itself was a gracious administration of the covenant. Why was the gracious covenant not believed? Because Israel was another Adam, not unusual for her unbelief but a microcosm of humanity as a whole. Only the Spirit can change the human heart, and the large-scale out-pouring of the Spirit is the distinct gift of the new covenant. The Spirit brought new birth under the Mosaic covenant, but only to a small number (think of Elijah’s 7000), hence the law-Spirit contrast in 2 Corinthians 3.