Often it is hard to appreciate the power of a book until you have read the last page. Similarly, it is often hard to measure the impact of a person's life until he or she passes on. Many of us felt the impact of the death of Timothy Keller. Now that he has passed on we can slowly begin to fully appreciate his life and ministry. It is hard to imagine what the church would be today without his gracious, biblical, balanced input. Those of us who did not follow his life closely were taken by surprise by his passing. Those who knew of his health concerns were less surprised. Still, his passing set off shock waves of various emotions throughout the church worldwide.
I, too, feel a deep sense of gratitude to this man and his work. When I heard of his passing it caused me to wonder: when my time comes, what sort of legacy would I leave behind? I am sure that the impact of my life would be so much less than his. But this is just me being silly. We remember again that Jesus said, “Well done, good and faithful servant!” (Matthew 25:23) He did not say, “Well done, good and successful servant”, or even, “Well done, good and effective servant”, but “Well done, good and faithful servant.”
God's measure of a life is the degree to which we have been faithful to the life entrusted to us. On the list of heroes of faith in Hebrews 11 are many familiar names; Abraham, Moses, and David, to name a few. But on that list are also people who did not see the fruit of their faith in this life. Indeed their names are not even mentioned:

There were others who were tortured, refusing to be released so that they might gain an even better resurrection. Some faced jeers and flogging, and even chains and imprisonment. They were put to death by stoning; they were sawed in two; they were killed by the sword. They went about in sheepskins and goatskins, destitute, persecuted and mistreated—the world was not worthy of them. They wandered in deserts and mountains, living in caves and in holes in the ground.
(Hebrews 11: 35–38, NIV)

What is clear is that these nameless heroes of the faith are no less important than the names we all know. And God knows them all by name and commends them for their faithfulness.
Among the many, many lessons we learn from Tim Keller is his clear focus to be who God wanted him to be, and to do what God wanted him to do. In the words of one tribute:

Jesus has loved the church with the unique voice of Dr. Tim Keller’s mind, heart, and soul. The bride of Christ is more beautiful and faithful because of Dr. Keller’s life and ministry. Well done good and faithful servant. — Jonathan Elgersma, Senior Pastor of Faith Reformed Church Zeeland

We are not called to be Tim Keller. But we are called to be as faithful as he was.