While Tim Keller has his faults, credit is given when credit is due. In this post over at Reformaton Theology, the centrality of the Gospel for Christians is well stated by Keller.
It is very common in Christian circles to assume that “the gospel” is something just for non-Christians. We presume that the gospel is a set of basic “A-B-C” doctrines that Christians do not need to hear or study once they are converted. Rather, they should move beyond the gospel to more “advanced” doctrines. But the great declaration of the gospel of grace in Galatians was written to believers who did not see the implications of the gospel for life-issues confronting them. Paul solves the disunity and racial exclusivity not with a simple exhortation to “be better Christians.” but by calling them to live out the implications of the gospel. So Christians need the gospel as much as non-Christians do. Their problems come because they tend to lose and forget the gospel. They make progress only as they continually grasp and apply the gospel in deeper ways.