Keller is commenting on a sermon by David Martyn Lloyd-Jones in the book Walking with God: Studies in 1 John (Crossway, 1993). The message deals with the tension between emphasizing doctrine over life experience or vice versa.
On the Danger of promoting experience over doctrine, Keller says:
"So when you say, “I don’t care about doctrine, it’s how you live that matters,” you are ironically promoting the doctrine of justification by works. You are proposing that what God really wants is a good life. The response can be similar when someone claims that it doesn’t matter which religion you belong to, because all religions are alike and no one should be held to a particular doctrine of God. Yet that assumes that God is not holy, and that he does not hold people responsible for how they live. In other words, to say, “No one should be held to a particular view of God” is to assume and promote a particular view of God. To say, “Doctrine about God doesn’t matter” is itself a statement of doctrine about God — and therefore it does matter! So Lloyd-Jones concludes: “It is no use your saying, ‘We are not interested in doctrine; we are concerned about life’; if your doctrine is wrong, your life will be wrong” (p. 23; italics added).
On the danger of promoting doctrine over experience, he says:
However, whenever Lloyd-Jones takes up the importance of doctrine, he always points out that there is a danger on the other extreme. He speaks of some Christians and says, “There is nothing they delight in more than arguing about theology” and they do this in “a party spirit” (p. 24). One of the signs of this group is that they are either dry and theoretical in their preaching, or they can be caustic and angry. They have “lost their tempers, forgetting that by so doing they were denying the very doctrine which they claimed to believe” (p. 24). In short, ministers who go to this extreme destroy the effectiveness of their preaching. What is the cause of this? Lloyd-Jones answers that they have made accurate doctrine an end in itself, instead of a means to honor God and grow in Christ-likeness. “Doctrine must never be considered in and of itself. Scripture must never be divorced from life” (p. 25).
That is some good and wise commentary.
Update: There is a Tim Keller Wiki resource page!