Book 8 defines indemnity as the way of re-creation. That shifts restoration away from a courtroom image of simple pardon and toward a workshop image of something broken being rebuilt.
The reasoning is straightforward: if the Fall damaged the original human being, then forgiveness alone cannot complete the task. The damaged humanity has to be re-formed by walking the reverse path of the Fall under God’s will.
This is why the chapter compares indemnity to bitter medicine. Recovery is harder than original formation because the ruin has to be undone as well as the original design restored.
The sermon force here is strong: salvation is not only that God stops condemning the old self. It is that He is committed to rebuilding what the Fall wrecked.