While few historic Christian beliefs are more ignored or misunderstood than the doctrine of the Trinity today, fewer still are more important or practical to the ongoing life of the church. Lately, I’ve been running across a strange new “angle” that sheds light on what Christians (should) mean when we say that God’s identity is constituted by the eternal relationship that exists between Father, Son and Holy Spirit. A little background first.
Karl Rahner, a prominent 20th-century Catholic theologian, is famous for his “rule” about thinking rightly about the Trinity: the economic Trinity is the immanent Trinity, and the immanent Trinity is the economic Trinity. I know, I know: this slogan makes you want to stop right here, turn aside, and starting worshipping God with unrestrained passion. Delay that explosive impulse for a moment to keep reading. What Rahner means by this very boring, typically dry academic statement is in fact–all appearances to the contrary notwithstanding–a crucial and beautiful reality that all Christians should know and treasure. In a word, Rahner is reminding us that the God who makes Himself known in Jesus is not arbitrary. Instead, He is 
deeply faithful and ever true in the manner He chooses to communicate with us, in all the assorted ways He reveals Himself to His creation. He never hits a false note or acts out of character. When the triune God interacts with His creatures in creating, sustaining, judging and redeeming the world, He does not put on a “mask” to hide what He is really like from us or play “peek-a-boo” with a hopelessly naive audience. Instead, the way that God interacts with us is dynamically consistent with the way He has always been in Himself. Jesus’ delightful, voluntary obedience to the Father did not start with the Incarnation, nor did the Spirit’s mediation of the Father’s pleasure in the Son begin at Jesus’ baptism. This is what God has always been like, with no beginning and no end. To summarize: when we see the ways in which the Father, Son and Holy Spirit relate to each other and to the world and the people of God in the Christian narrative, Rahner insists (rightly) that we can be confident that these very same character traits have always been present within God’s own triune life.
But perhaps a further, even more radical suggestion can be made. What would it look like for the God who is characterized by self-giving, mutually reciprocated love to enter a fallen, self-centered world? If the God of Jesus Christ has always been the kind of God who puts the interest of others ahead of His own, and who is not self-seeking, false or manipulative–well, what happens when that sort of person shows up in the flesh? It’s not hard to figure out; the gospels tell us. Rejection. Humiliation. Crucifixion. This is self-giving, others-centered love brought to its climactic fulfillment. Jesus, in putting the interests of others ahead of his own, acted out in the Incarnation and on the Cross what, in a very real sense, he had already been doing forever (Philippians 2:5-11).
Yet if God the Father loves the Son with infinite desire and committment, then He cannot let that horror be the final act of the drama. And so, through the Spirit, He raises His beloved Son from the grave. But this time, He brings us along for the ride, to enter into God’s own life. Therefore, the gospel story is a fitting enactment of God’s own triune life writ large on the canvas of a world gone astray in sin and evil, by overcoming evil with good. Consider these wondrous claims. And this time, I really do hope this moves you to exalt in the matchless glory of our triune God who makes Himself known in the face of Jesus Christ:
“In self-giving, if anywhere, we touch a rhythm not only of all creation but of all being. For the Eternal Word also gives Himself in sacrifice; and that not only on Calvary. For when He was crucified He ‘did that in the wild weather of His outlying provinces which He had done at home in glory and gladness.’ From before the foundation of the world He surrenders begotten Deity back to begetting Deity in obedience. And as the Son glorifies the Father, so also the Father glorifies the Son…There is joy in the dance, but it does not exist for the sake of joy. It does not even exist for the sake of good, or of love. It is Love Himself, and Good Himself, and therefore happy. It does not exist for us, but we for it.” (C. S. Lewis, The Problem of Pain, pp. 136-38)
And:
“The story of Jesus is what the eternal trinitarian life of God looks like when it is projected upon the screen of history, and this means on the screen not only of human history but of sinful human history. The obedience of Jesus to the Father, his obedience to his mission, is just what the eternal procession of the Son from the Father appears as in history. His obedience consists in nothing else but being in history, in being human. Jesus did nothing but be the Son as man; that his life was so colorful, eventful and tragic is simply because of what being human involves in our world. We for the most part shy off being human because if we are really human we will be crucified. If we didn’t know that before, we know it now; the crucifixion of Jesus was simply the dramatic manifestation of the sort of world we have made, the showing up of the world, the unmasking of what we call, traditionally, original sin…
The crucifixion in this sense is the supreme expression of Jesus’ humanity…the supreme expression of his obedience to the Father, of his eternal Sonship. On the cross he casts himself simply on the Father. It is his prayer to the Father, the only prayer known to Christians, and the resurrection is the Father’s response…
And this communication of eternal prayer and response is what the Holy Spirit is–which is why Jesus speaks of sending the
Holy Spirit in history when he is united with his Father. Just as the crucifixion/resurrection is what the eternal procession of the Son from the Father looks like when projected upon sinful human history, so the sending of the Holy Spirit (so that we share in the life of God, so that the mystery of the Church exists) is what the eternal procession of the Holy Spirit looks like when projected on to that sinful human world. And the Holy Spirit appears in our world of course as catastrophic and destructive, as a revolutionary force making the world new, or the Church new, the individual new, by reducing them first to chaos.
That (I’m afraid) is a very compressed sketch of what the Christian means to be saying when he speaks of God as Trintiy. And in the end what it all boils down to is this central mystery that God is love.” (Herbert McCabe, God Matters, pp. 22-23)