Dane Ortlund writes at the Crossway Blog:
After all, after 2,000 years, don't we know by now what the gospel is? Haven't we "been-there-done-that?" Why do we need one book after another on the same old topic?
1. Because the gospel is "of first importance" (1 Cor 15:3).
In describing his ministry—a ministry that communicated "the whole counsel of God" (Acts 20:27)—Paul described it as testifying "to the gospel of the grace of God" (Acts 20:24).
2. Because you're going to roll out of bed tomorrow a functional Pharisee.
The instincts beneath your instincts, the impulses way down deep inside you, are law, not gospel. A good night's sleep, not a heretical sermon, is all it takes to forget the gospel of grace.
3. Because the gospel is disputed and debated today.
What is the gospel? What are the implications of the gospel? What is the relationship between the gospel and the kingdom of God? How does the gospel relate to growth in godliness? What is the connection between the gospel and community? These questions need answers from different people, with different voices and different backgrounds, who love the same gospel.
4. Because the church is always one generation away from losing the gospel.
Every generation must rediscover the glories of free grace for itself.
5. Because for every book exulting in or explaining or defending the gospel, a hundred more roll off the press which, wittingly or unwittingly, distract us from that which is of first importance.
6. Because the gospel is the central message of the entire Bible.
Jesus said that even Moses was writing, ultimately, about him (John 5:46). The last verse of the Bible sums up the core message of the Bible: "The grace of the Lord Jesus be with all. Amen" (Rev. 22:21).
The gospel is the scandalous news that through the death and resurrection of Jesus, our disobedience cannot dent God's approval of us and our obedience cannot help God's approval of us, as we look in trusting faith to Christ. And the priority of this gospel, the functional need of the gospel, the contesting of the gospel, the retaining of the gospel, the constant sidelining of the gospel, and the unified biblical testimony to the gospel all unite to say—yes, we need more books on this gospel.