Most Christians know the trinitarian formula: God the Father, the Son, and the Holy Spirit. But Moltmann emphasizes the importance of viewing God not just as the "Father", but as the Father *of the Son*; Christ not just as Son, but as *the Father's Son*; and Spirit not merely as bond of their love but co-equal person with its own granted activity in the Godhead. In short, he returns to the significance of the *perichoretic* union of the Godhead (as it was first propounded by Church Father Tertullian and Maximos the Confessor) rather than just the substantial union (which medieval scholasticism emphasized to some detriment). Viewing God as merely an absolute subject or a supreme substance, in JM's view, ultimately depersonalizes this affected Father, affected Son, affected Spirit in its complex unity, and JM is (I think) again correct that one is better off interpreting God's nature by starting with the trinity--the threeness--rather than with the unity--the oneness. Starting with the One, as has been pointed out through centuries of Christian dialogue, leads more readily to violent misunderstandings of God's character (i.e., the angry monotheistic monarch) that leave God distant, apathetic and unfeeling, if not just terrifying. Some quotes here demonstrate the power of this trinitarian corrective, which is really just a recovery of Christian tradition that leans more on the Eastern Fathers:
"Where the theological perception of God and history is concerned, there will be a modern discovery of trinitarian thinking when there is at the same time a fundamental change in modern reason--a change from lordship to fellowship, from conquest to participation, from production to receptivity. The new theological penetration of the trinitarian history of God ought also to free the reason that has been made operational--free it for receptive perception of its Other, free it for participation in that Other. Trinitarian thinking should prepare the way for a liberating and healing concern for the reality that has been destroyed." (p. 9)
The significance of contemplation, of wonder in the life of the believing practitioner, is lost when the Godhead is left as a sameness-in-Himself--just a moral idea or a higher substance--and not a fellowship of unified persons. Contemplating the triune relationship of the Godhead is a way of combating the more meritocratic, capitalistic social doctrine of the day which often unhelpfully views knowledge not as a gateway to experiencing others, but as a way of possessing (power over) others. God can be more easily objectified and grasped as a sameness, a singular idea, than as a diversity-in-Himself (AKA, a triunity), which is not necessarily a danger to God, but a danger to ourselves. As JM says, "The reduction of faith to practice has not enriched faith; it has impoverished it. It has let practice itself become a matter of law and compulsion. If we are to be freed *for* practice--not from it!--it is important for meditation, contemplation and doxology to be rediscovered." (p. 8)
I'll end on some beautiful notes from the rest of the book:
"God is love. That means God is self-giving. It means he exists for us: on the cross. To put it in trinitarian terms--the Father lets his Son sacrifice himself through the Spirit. 'The Father is crucifying love, the Son is crucified love, and the Holy Spirit is the unvanquishable power of the cross. The cross is at the center of the Trinity. This is brought out by tradition, when it takes up the Book of Revelation's image of 'the Lamb who was slain from the foundation of the world' (Rev. 5:12). Before the world was, the sacrifice was already in God."
"What the love of God is--the love 'from which nothing can separate us' (Rom. 8:39)--becomes event on the cross and is experienced under the cross. The Father who sends his Son through all the abyss and hells of God-forsakenness, of the divine curse and final judgment, is, in his Son, everywhere with those who are his own; he has become universally present. In giving up the Son he gives up 'everything' and 'nothing' can separate us from him. This is the beginning of the language of the kingdom of God, in which 'God will be all in all'. Anybody who perceives God's presence and love in the God-forsakenness of the crucified Son, sees God in all things, just as, once having faced the experience of death, a person feels the living character of everything in a hitherto undreamed of way." (p. 82-83)
"The creation of the world... is a moment of the deepest mystery in the relation between God the Father and God the Son.' Creation is a part of the eternal love affair between the Father and the Son. It springs from the Father's love for the Son and is redeemed by the answering love of the Son for the Father. Creation exists because the eternal love communicates himself creatively to his Other. It exists because the eternal love seeks fellowship and desires response in freedom. That is why we have indeed to see the history of creation as *the tragedy of divine love*, but must view the history of redemption as *the feast of divine joy*." (p. 59)
".. the tragedy of human history is God's own tragedy too. God desires the freedom of his image on earth, and yet cannot force freedom on him; he can only create it and preserve it through the suffering of his eternal love." (p. 42)
"The sole omnipotence which God possesses is the almighty power of suffering love. It is this that he reveals in Christ. What was Christ's essential power? It was love, which was perfected through voluntary suffering; it was love, which died in meekness and humility on the cross and so redeemed the world. This is the essence of the divine sovereignty." (p. 31)
"The expression 'experience of God' therefore does not only mean our experience of God; it also means God's experience with us. (...) God suffers with us--God suffers from us--God suffers for us." (p. 4)
There was also a rather lovely meditation on creation: something like, "Through Christ the Son, The Father saw the world."
In sum: this book holds a lot of thoughtful material on the subject (the bibliography is a great resource) and is worth the read. Some possible issues of process theology, open theism, and filioque come up quite naturally from time to time, but these are probably better adjudicated in light of Moltmann's holistic corpus, which I have not read.