Long ago, and far far away, I set out with the good intention of summarising Oliver O'Donovan's book of the above title. Having completed a summary of chapters 1 and 2 I got lost in time and inertia. Meanwhile, however, I have discovered a good brief abstract of the book by David vanDrunen in 'Natural Law and the Two Kingdoms'. It is a footnote on page 431 (yes, it is one of those books with half page footnotes). So here is vanDrunen's precis of O'Donovan, slightly abridged by me.
Remember that vanDrunen is approaching O'Donovan from the perspective of the latter's handling of the 'two kingdoms' doctrine in Reformed theology, but it is nonetheless a helpful outline of O'Donovan's thesis.
O'Donovan acknowledges something of a two kingdoms reality in parts of the Old Testament, but he emphasizes Christ's proclamation of the kingdom which announces the unity of the religious and political realms under the reign of God and challenges the two kingdoms situation (ch.3).
In Christ's resurrection, the earthly powers have been subdued and made subject to divine sovereignty; yet the sovereignty of God is not now completely manifest, and the powers are still given a certain (secular) space and authority to exercise their judicial function, though they ought to serve the church's mission (ch. 4).
After Christ's ascension, therefore, the terms on which political authorities function are not the same as they were before; see also O'Donovan's 'The Ways of Judgment' (Eerdmans 2005), p 5. Society is to be transformed and its rulers disappear. Christendom (" the idea of a confessionally Christian government") is not a project of, but a response to, the church's mission, as the alien powers become attentive to the church. The Christian state may be disclosed from time to time but it should not coerce belief or try to protect its own existence (ch.6).
In 'The Ways of Judgment' he speaks of the redemptive, transforming work of the church, gospel, and Holy Spirit on the state as the sphere of human judgment and therefore argues that there is a place for mercy in civil judgement (ch. 6). Here he also discusses the proclamation of the cross and the coming of the kingdom as a challenge to the conditions of the earthly political authority and opposes an a-political theology disinterested in social life (231-34).
From the other direction, in 'The Ways of Judgment', he critiques the two swords idea, originating with Gelasius, for teaching that there are certain spheres of social life that are in principle beyond the reach of governmental intervention (62).