"There is an incredible amount of confusion even among people who would call themselves Evangelicals—about what the content of the Christian gospel is," explains Greg Gilbert, author of What is the Gospel. "If you ask 100 Evangelicals, 'What is the Gospel?,' you're going to get 60-70 different answers."
So how do we define it? Gilbert turns to the Bible and starts by considering Paul's argument in Romans 1-4 to identify four essential questions at the heart of the proclamation of the gospel:
Who made us, and to whom are we accountable?
What is our problem? In other words, are we in trouble and why?
What is God’s solution to that problem? How has he acted to save us from it?
How do I—myself, right here, right now—how do I come to be included in that salvation? What makes this good news for me and not just for someone else?
We might summarize these four major points like this: God, man, Christ, and response. Of course Paul goes on to unfold a universe of other promises God has made to those who are saved in Christ, and many of those promises may very appropriately be identified as part of the good news of Christianity, the gospel of Jesus Christ. But it’s crucial that we understand, right from the outset, that all those grand promises depend on and flow from this, the heart and fountainhead of the Christian good news. Those promises come only to those who are forgiven of sin through faith in the crucified and risen Christ. That is why Paul, when he presents the heart of the gospel, starts here—with these four critical truths.
Modified from What is the Gospel? by Greg Gilbert.