Mike Horton writes about the importance of Gospel-centered preaching in his book Justified: Modern Reformation Essays on the Doctrine of Justification:
Yet the greatest treasure buried in the Scriptures is the good news of the promised Messiah. Everything in the Bible that tells us what to do is “law”, and everything in the Bible that tells us what God has done in Christ to save us is “gospel.”
Much like medieval piety, the emphasis in much Christian teaching today is on what we are to do without adequate grounding in the good news of what God has done for us in Christ. “What would Jesus do?” becomes more important than “What has Jesus done?”
The gospel, however, is not just something we needed at conversion so we can spend the rest of our Christian life obsessed with performance; it is something we need every day–the only source of our sanctification as well as our justification. The law guides, but only the gospel gives. We are declared righteous–justified–not by anything that happens within us or done by us, but solely by God’s act of crediting us with Christ’s perfect righteousness through faith alone.