From Tullian Tchividjian's new book Surprised by Grace:
I once assumed the gospel was simply what non-Christians must believe in order to be saved, but after they believe it, they advance to deeper theological waters. Jonah helped me realize that the gospel isn’t the first step in a stairway of truths but more like the hub in a wheel of truth. As Tim Keller explains it, the gospel isn’t simply the ABCs of Christianity, but the A-through-Z. The gospel doesn’t just ignite the Christian life; it’s the fuel that keeps Christians going every day. Once God rescues sinners, his plan isn’t to steer them beyond the gospel but to move them more deeply into it. After all, the only antidote to sin is the gospel—and since Christians remain sinners even after they’re converted, the gospel must be the medicine a Christian takes every day. Since we never leave off sinning, we can never leave the gospel.
This idea that the gospel is just as much for Christians as for non-Christians may seem like a new idea to many, but, in fact, it is really a very old idea. In his letter to the Christians of Colossae, the apostle Paul quickly portrays the gospel as the instrument of all continued growth and spiritual progress for believers after conversion: “All over the world,” he writes, “this gospel is bearing fruit and growing, just as it has been doing among you since the day you heard it and understood God’s grace in all its truth” (Col. 1:6 NIV).
After meditating on Paul’s words here, a friend once told me that all our problems in life stem from our failure to apply the gospel. This means we can’t really move forward unless we learn more thoroughly the gospel’s content and how to apply it to all of life. Real change does not and cannot come independently of the gospel, which is the good news that even though we’re more defective and lost than we ever imagined, we can be more accepted and loved than we ever dared hope, because Jesus Christ lived, died, and rose again for sinners like you and me. God intends this reality to mold and shape us at every point in every way. It should define the way we think, feel, and live.
Martin Luther often employed the phrase simul Justus et peccator to describe his condition as a Christian. It means “simultaneously justified and sinful.” He understood that while he’d already been saved (through justification) from sin’s penalty, he was in daily need of salvation from sin’s power. And since the gospel is the “power of God for salvation” (Rom. 1:16), he knew that even for the most saintly of saints the gospel is wholly relevant and vitally necessary—day in and day out. This means that heralded preachers need the gospel just as much as hardened pagans. (16-17)