In Mysterium Salutis Karl Rahner enunciates a simple formula that should be regarded as axiomatic for all mediatation upon the Christian doctrine of God: "The 'economic' trinity is the 'immanent' trinity and the 'immanent' trinity is the 'economic' trinity." The modern return of Western systematic theology to the doctrine of the Trinity--to many eyes, the most metaphysical of Chrsitian credenda--has been the result, in point of fact, of a renewed and earnest attention to the particularities of Christian history, the concrete details of the story of Christ and his church, and the scriptural understanding of how God has acted within history for the restoration of the created world. The pathology of liberal Protestant theology's dogmatic wasting disease--of which no symptom could be more acute that the reduction of the doctrine of the Trinity to an appendictic twinge at the end of Schleiermacher's The Christian Faith --was one of progressive and irrepressible abstraction, a moralization and spiritualization that made of Christ the unique bearer (as opposed to the unique content) of the Christian kerygma; and the theological rediscovery of the Trinity has come about precisely because the salvific significance of Christ's historical specificity has been to some considerable degree recovered from the confining prejudices of modern thought.... In the early centuries of Christian thought, the Trinity was gradually apprehended as the mystery truly revealed in God's saving action, and not as a metaphysical secret imparted mystically to the church; it was not until three centuries and more had elapsed, councils had been called, and doctrine had been defined that a text like Augustine's De Trinitate, in which the doctrine assumed the aspect of faith's object (as opposed to its explication), was possible: and while such a possibility, in one sense, was the result of a certain ecclesial liberation from the anxiety of dogmatic dispute, in another sense it was arguably the occasion for the inauguration of a certain pattern of theological forgetfulness. Not that there is anything to be deplored in Augustine's magnificent series of brilliant, intense, and theologically necessary theoretical allegorieson the doctrine of the Trinity, but it is also the case that to some eyes they are not always obviously wedded to a deeper consideration of the story of the atonement. And later theology (Eastern no less than Western) sometimes obeyed the logic of the divorce of the doctrine of God from the story of God's manifestation of himself in history--with occasionally dismal consequences. Trinitarian thought uninformed by the gospel narrative results, inevitably, in an impoverishment of both that thought and that narrative; hence the importance of the affirmation that the Trinity as economic or immanent is the one God as he truly is, whose every action is proper to and expressive of his divinity.
The Beauty of the Infinite, pp. 155-156