Greg Gilbert answers this question in his book What Is The Gospel? The following are some quotes from this much needed piece of literature:
“Let me suggest that, for now, we approach the task of defining the main contours of the Christian gospel not by doing a word study, but by looking at what the earliest Christians said about Jesus and the significance of his life, death, and resurrection. If we look at the apostles’ writings and sermons in the Bible, we’ll find them explaining, sometimes very briefly and sometimes at greater length, what they learned from Jesus himself about the good news. Perhaps we’ll also be able to discern some common set of questions, some shared framework of truths around which the apostles and early Christians structured their presentation of the good news of Jesus” ( P. 26-27 Kindle Edition).
“By the middle of chapter 3, Paul has indicted every single person in the world with rebellion against God. “We have already charged that all, both Jews and Greeks, are under sin” (v. 9). And his sobering conclusion is that when we stand before God the Judge, every mouth will be silenced. No one will mount a defense. Not one excuse will be offered. The whole world—Jew, Gentile, every last one of us—will be held fully accountable to God (v. 19). Now, strictly speaking, these first two points are not really good news at all. In fact, they’re pretty bad news. That I have rebelled against the holy and judging God who made me is not a happy thought. But it is an important one, because it paves the way for the good news. That makes sense if you think about it. To have someone say to you, “I’m coming to save you!” is really not good news at all unless you believe you actually need to be saved” (p. 29 Kindle Edition).
“Third, Paul says that God’s solution to humanity’s sin is the sacrificial death and resurrection of Jesus Christ. Having laid out the bad news of the predicament we face as sinners before our righteous God, Paul turns now to the good news, the gospel of Jesus Christ. “But now,” Paul says, in spite of our sin, “now the righteousness of God has been manifested apart from the law” (v. 21). In other words, there is a way for human beings to be counted righteous before God instead of unrighteous, to be declared innocent instead of guilty, to be justified instead of condemned. And it has nothing to do with acting better or living a more righteous life. It comes “apart from the law.” So how does it happen? Paul puts it plainly in Romans 3:24. Despite our rebellion against God, and in the face of a hopeless situation, we can be “justified by his grace as a gift, through the redemption that is in Christ Jesus.” Through Christ’s sacrificial death and resurrection—because of his blood and his life—sinners may be saved from the condemnation our sins deserve” (p. 29-30 Kindle Edition).
“Faith and repentance. This is what marks out those who are Christ’s people, or “Christians.” In other words, a Christian is one who turns away from his sin and trusts in the Lord Jesus Christ—and nothing else—to save him from sin and the coming judgment” (p. 73 Kindle Edition).
“An emaciated gospel leads to emaciated worship. It lowers our eyes from God to self and cheapens what God has accomplished for us in Christ. The biblical gospel, by contrast, is like fuel in the furnace of worship. The more you understand about it, believe it, and rely on it, the more you adore God both for who he is and for what he has done for us in Christ