Appropriately, Tim Keller ends The Reason for God with a chapter on the Trinity. If I was investigating Christianity as a skeptic I'm pretty sure the doctrine I would have the most trouble with would be the doctrine of the Trinity. One God in three persons. Really?! All that business about essences and substances just seems like splitting hairs, right? On the face of it the doctrine seems like a logical impossibility. Aren't our Jewish and Muslim neighbors right when they accuse Christians of being polytheists? After all, the main thrust of the Old Testament seems to be monotheism. Nothing about the Trinity there.
As you probably guessed I'm playing a bit of devil's advocate. No, I don't believe Christians are polytheists, and one can make out the beginnings of trinitarian theology even in the OT. Nevertheless we must acknowledge that we're in the presence of a mystery we can't fully explain. Keller writes: "The doctrine of the Trinity overloads our mental circuits." We may never wrap our minds around it, but the Trinity is essential to Christianity. So much flows from it, not least the Incarnation of the Son of God that we celebrate in a few days.
Unique to the Christian understanding of God is that he is love. It's not only that he is loving (which he is!), but that he IS love. From eternity past self-giving love has been part of the essence of the Godhead -- Father, Son and Holy Spirit. This would not be possible if God wasn't triune, Keller explains.
If God is unipersonal, then until God created other beings there was no love, since love is something that one person has for another. This means that a unipersonal God was power, sovereignty, and greatness from all eternity, but not love. Love then is the essence of God, nor is it at the heart of the universe. Power is primary.
However, if God is triune, then loving relationships in community are the "great fountain . . . at the center of reality." When people say, "God is love," I think they mean that love is extremely important, or that God really wants us to love. But in the Christian conception, God really has love as his essence. If he was just one person he couldn't have been loving for all eternity. If he was only the impersonal all-soul of Eastern thought, he couldn't have been loving, for love is something persons do. . . . Ultimate reality is a community of persons who know and love one another.
If Keller is right, and the Trinitarian nature of God is the key to ultimate reality, then this has massive implications for the way we live. For one thing it shows us that relationships are key to human flourishing, and selfishness is ultimately a destructive dead end. Jesus put it most succinctly: "For whoever wants to save their life will lose it, but whoever loses their life for me and for the gospel will save it" (Mark 8:35 NIV). In sum: "You were made for mutually self-giving, other-directed love. Self-centeredness destroys the fabric of what God has made."
The Reason for God is terrific. Even better than I expected. One of the author's heroes is C.S. Lewis, so it's not surprising that his fingerprints are all over the book. I think Keller has written a Mere Christianity-type book for our day -- one that will strengthen the faith of believers, respectfully challenge skeptics, and reward sincere seekers of truth.
Quotes from The Reason for God: Belief in an Age of Skepticism (pp. 225-7)