In The Trinity, Karl Rahner laments the lack of scholarship put “towards development within [trinitarian theology]”. (The Trinity, p. 9) Positive work in trinitarian theology has fallen so far by the way that “should the doctrine of the Trinity have to be dropped as false,” Rahner says, “the major part of religious literature could well remain virtually unchanged”. (pp. 10-11) The doctrine of the Trinity, it seems, bears little to not at all on the Christian life. This stems, in part, from an imposed disjunction of theological loci in dogmatics: namely, the disjunction between the treatises “On the One God” and “On the Triune God”. Progress in trinitarian theology can be achieved in part seeing again the mutual indwelling of these two treatises.
Rahner’s fundamental thesis is as follows: “The ‘economic’ Trinity is the ‘immanent’ Trinity and the ‘immanent’ Trinity is the ‘economic’ Trinity.” (p. 22) In other words, the God who has revealed himself in salvation history as Father, Son, and Holy Spirit is the one true God, and this God truly is, within the divine life, as he has revealed himself. We know what we know of God through his self-revelation, and he has revealed himself in the Word by the Spirit.
While addressing difficulties connected with his thesis, Rahner states a presupposition that seems fundamental to his project:
“We develop a theology which neither explicitly nor (more dangerously) implicitly considers a pretended possibility never mentioned in revelation; we cling to the truth that the Logos is really as he appears in revelation, that he is the one who reveals to us (not merely one of those who might have revealed to us) the triune God, on account of the personal being which belongs exclusively to him, the Father’s Logos.” (p. 30)
Rahner makes this statement in respect, particularly to the question of the uniqueness of incarnation of the Son, ruling out an ‘incarnational potentiality’ in the divinity in general. More broadly, though, this presupposition carries through the rest of his thought: that what we see of God in salvation history matches what is true of God in himself, ontologically.