My rating: 3 of 5 stars
Moltmann's engagement with Trinitarian theology is highly thought-provoking. One thing I greatly appreciated was his insistence that, following the biblical writers, we must begin by talking about the threeness of God and then move toward oneness. In line with this, I think he successfully demonstrated the inadequacy of some western formulations of the trinity that consider it sufficient to say that the different persons all share the same 'divine substance,' as though this were all that mattered. (In my mind, this can tend toward tri-theism even more than Moltmann's 'social' model of the Trinity, since it doesn't lay enough stress on the mutual indwelling of the persons in one another - they are simply three beings that hold a certain attribute in common.) Perhaps the most engaging portion of the book is in its discussion of 'the passion of God,' i.e. the question of how the triune God of Christian confession relates to the problem of evil and suffering in the world. In keeping with his strong theology of the cross (best exemplified in his masterpiece, "The Crucified God"), Moltmann insists that we regard suffering love as intrinsic to the very identity of God, such that a creation in which the Son did not offer himself up to death would be inconceivable.
With reference to the title, Moltmann's book is 'heavy' on trinity and 'light' - extremely light - on kingdom. He doesn't even attempt sufficiently to ground his ideas within the ancient biblical concept of the kingdom of God; he simply uses the term as though it manifestly means what he thinks it does (which it manifestly doesn't, at least not if he intends to mean by it something like what the biblical writers of the second temple period meant by it). His general inattentiveness to the biblical text and ancient historical context at this point (as at others) is hampered further by his modern allergy to any hint of hierarchy whatsoever, which in my mind yields a skewed conception of the kingdom of God that demands we sweep large portions of the biblical text - both in the gospels and in the epistles, as well as vast swathes of the Old Testament - under the rug. In tracing the connections between a monarchical monotheism and human systems of oppression, he throws the baby out with the bathwater, assuming that the problem is with the notion of power and authority per se and not in our human perversion of them in disobedience to the good authority of a loving God. The great hope of the biblical writers seems to be that God would be acknowledged as king on earth as in heaven, and that his will would be done and the knowledge of him would be perfected; only the Reign of God, on earth as in heaven, could yield true creaturely freedom. (This latter point, in fairness, is something Moltmann recognizes - he acknowledges that true 'freedom' entails not only an openness of decision but specifically a willingness for the Good.)
Moltmann's ideal, however, leaves us with a God who is not a king but a friend (as though the two were mutually exclusive!). His argument depends on a softening of the biblical concepts of fatherhood and sonship, as well as a total ignoring of the idea of headship, such that he emerges with a totally egalitarian reading of the Trinity that seems to leave little room for Paul's affirmation, precisely on the point of authority and submission, that "the head of Christ is God" (1 Cor 11:3) and that the Son to whom all things have been subjected will himself be subjected to the Father in the age to come such that the God to whom all is subjected will at last be "all in all" (1 Cor 15:27-28). Where God's status as "all in all" for Moltmann entails something like Christian panentheism, for Paul it clearly means exactly what Moltmann, despite using his phrase, does not want it to mean - namely, that the entire created order is, at last, fully in unmediated submission to its Creator King. In the final analysis, Moltmann emerges with a picture of the age to come that, while interesting and certainly appealing to modern western liberal democrats, does not appear to resemble clearly the biblical picture of God become king in Jesus of Nazareth, and implementing his rule through the Holy Spirit.
Moltmann thus escapes a heterodox subordinationism, but ends up denying any form of hierarchy whatsoever within the Godhead in the process. The biblical witness speaks strongly otherwise; this needn't equate to hierarchical systems within the church as he fears, moreover, since 'monarchical monotheism' (as Moltmann calls it) is precisely the thing that is meant to keep hierarchical relationships in check (as with slaves and masters) because both slave and master, both husband and wife, both parent and child, and both Jew as the gospel's first recipient and Gentile as equal second are in every case equally answerable to the very same Lord and God over all. Slaves and masters alike have a Master in heaven; husbands and wives are all alike under the headship of the same Bridegroom; parents and children, Jews and Gentiles, are all alike the children of one heavenly Father. When God is all in all, there can be no oppression because this is not how he rules the world; domination between God's creatures suggests a lack of subjection to God the king, not a genuine recognition of him as such.
At this point, though, I've strayed more into philosophical differences with Moltmann than with anything wrong with the book. Apart from the imbalance between his discussion of the Trinity and of the Kingdom (with a particular dearth of textual engagement when it comes to the latter), this book is an excellent, engaging inquiry into perhaps the greatest mystery of the Christian faith, and I highly recommend it to anyone interested in the topic. If you are looking to delve into Moltmann for the first time, however, a better place to start would be "The Crucified God."