Since I preached on the gospel last week, and this week we are talking about the extent of the gospel, I thought it would be worthwhile to spend some time thinking about what the gospel is at a more general level. And this is an important question because it is tempting to understand the gospel as so expansive that it encompasses everything (and sometimes everyone). D.A. Carson captures this temptation well, and why it is important to address:
Today it is very common to hear that such-and-such a topic is “a gospel issue.” We must hold to the eternal generation of the Son: it is a gospel issue. We must defend inerrancy: it is a gospel issue. We must espouse complementarianism: it is a gospel issue. … Alternatively, the weight of some doctrines may be diminished by our pronouncements if we declare that something or other is not a gospel issue. We then hear statements like these: Inerrancy may be important, but it is not a gospel issue. I disagree with your understanding of the role of the nation of Israel in the history of redemption, but that’s all right: it’s not a gospel issue. Why do you make such a fuss over complementarianism? After all, it’s not a gospel issue.
Speaking of something as a gospel issue has the impact of defining the gospel in this way of speaking. But if the gospel is everything, then it really is nothing. Alternatively, if the gospel is next to nothing, then it has little significance. That said, our striving for an explanation of the gospel must neither be overly narrow nor overly broad.
In my sermon last week, I said multiple times that there is no other gospel than the grace of the Lord Jesus. While I’m content with that as a shorthand description of the gospel, it’s worthwhile to say more. And we can begin with the Gospel of Mark, where we see that the whole of the life and work of Christ is the gospel. That is, after all, the implication of the opening verse of Mark: “The beginning of the gospel of Jesus Christ, the Son of God” (Mark 1:1 ESV). “For Mark, the gospel is the story of salvation in Jesus.” But this story of salvation is not a new one. “For Mark, the gospel refers to the fulfillment of God’s reign and salvation in the fullness of time (Isa 52:7; 61:1).”
Looking back to the roots of this good news, Isaiah declares, “How beautiful upon the mountains are the feet of him who brings good news, who publishes peace, who brings good news of happiness, who publishes salvation, who says to Zion, ‘Your God reigns’” (Isa 52:7 ESV). This verse lays out four characteristics of the good news that is proclaimed to God’s people. First, true peace has come. “There is peace to be proclaimed, for the Lord no longer stands in a hostile relation to His own.” Goodness and salvation then follow because of the fourth characteristic, which is the proclamation that God reigns.
Taking these two passages together, it is clear that the gospel is not only centered on Jesus but is Jesus himself. He is salvation; he is goodness; he is our peace (Eph 2:14) because he reigns.
But Christ’s manifestation of the kingdom of God is not really enough because the good news, according to Christ himself, is all about grace. Importantly, when Jesus inaugurates his public ministry, he quotes from Isaiah 61:1-2a. The good news is summarized as this: the year of the Lord’s favor. But Isa 61:2 continues, “and the day of vengeance of our God; to comfort all who mourn” (Isa 61:2 ESV). That is to say, the good news that the kingdom of God is at hand in the person and work of Jesus Christ is that the gracious reign of Christ has come. His judgment, strictly speaking, is not a part of the gospel. For this reason, I am content with my shorthand description of the gospel as the grace of the Lord Jesus.
There is one implication of this definition of the gospel that I want to highlight: our response is not the gospel. Our faith, repentance, and good works are not the gospel but the fruit of the gospel. This is important because it preserves the focus on the Lord Jesus as the sole content of the gospel message.
So, if you’re ever asked what the gospel really is, a good shorthand description is this: it’s the grace of the Lord Jesus. From this short phrase you can unpack the riches of God’s salvation to the praise of his glorious grace.
 D. A. Carson, “What Are Gospel Issues?,” Themelios 39.2 (2014): 215.  James R. Edwards, The Gospel According to Mark, PNTC (Grand Rapids: Leicester: Eerdmans; Apollos, 2002), 24.  Edwards, Mark, 25.