Let me explain. In Scot McKnight's book The King Jesus Gospel, he argues that we've misused the term "gospel." McKnight notes that when people talk about sharing the gospel or giving the gospel presentation, they often mean something like the Romans Road or Four Spiritual Laws. It's a message aimed at the individual, focusing especially on how Jesus' sacrifice takes away their individual guilt and provides them with eternal life, sometimes explained as "living in heaven forever." (I call this the "Transaction Jesus Gospel.")
McKnight puts the brakes on here. The term "gospel" should be used, he argues, to talk about the story of Jesus as fulfilling/resolving Israel's story. That's why the actual Gospels according to Matthew, Mark, Luke, and John record what they do: not a plan of personal salvation, but the story of Jesus, which only makes sense as resolving Israel's story (summarized succinctly in Deuteronomy 28-30 as covenant, exile, and restoration). McKnight doesn't deny the need for personal faith and repentance, the reality of justification by faith, etc. But the gospel, properly understood, is the story of the life, death, resurrection and ascension of Jesus. In Jesus, Israel's story has reached its long-awaited climax, as further confirmed by the outpouring of the promised Spirit.
Here's where Gal. 2:14 thoroughly convinces me of McKnight's point about the term gospel and how it should be used. When Paul says "they were not acting in line with the truth of the gospel," what is he talking about? Well, Peter and others had reverted back to eating apart from Gentile followers of Jesus. It's pretty obvious from the preceding verses that Peter was clearly not promoting something like "salvation by works" as a way to get one's individual sins forgiven (as you might expect if "gospel" is shorthand for "justification by faith" or "individual salvation"). But what's the big deal about eating with Gentiles? Why is that so closely connected with the gospel, understood as the story of Jesus fulfilling/resolving Israel's story?
When God ends Israel's exile and forgives their sins (a theme repeated over and over in the prophets), Gentiles will join together with Israel in the worship of the one true God, Israel's God. Just one example of this is Acts 15:16-18. James quotes Amos 9:11-12, which points to (a) God's restoration of Israel from exile and (b) Gentiles now bearing God's name. Now put McKnight's definition of gospel into Paul's sentence. Then you get: "they were not acting in line with the truth of how Jesus fulfills and resolves Israel's story." This fits exactly with how the early church interprets what is going on. Jew and Gentile are now united as one body in the Messiah, the one who brought about forgiveness of sins, the one who is Lord, and the one whose Spirit dwells within them. The promise of the prophets has been fulfilled in Jesus. For Peter and other Jews to go back to eating separately is saying: the Messiah hasn't really come. No wonder Paul gets so worked up! See, Paul's opponents are not medieval Catholics who are selling indulgences or trying to work their way into a heavenly afterlife; his opponents are Jews who are acting like the Messiah hasn't come yet, as evidenced in their eating habits!
Now, since it's MLK day, let's think about a changed understanding of "gospel" and how it affects something like racism and the reality of functional segregation in many American churches. For more conservative churches, where "gospel" is primarily about how the individual receives God's grace, then racism as an issue is not directly connected to the gospel. For the individual-centered, "Transaction Jesus" gospel, overcoming racism might be an implication of the gospel, but it has nothing to do with the gospel per se. Intentionally seeking to overcome racism is then seen as optional. If a church can get around to it, it would be nice, but it is not in any way connected to the core of the gospel. But even churches who are intentionally anti-racist often do so for reasons unconnected with the gospel (again: understood as how Jesus fulfills/resolves Israel's story). They might seek to be anti-racist based on generic American cultural ideals such as tolerance or diversity, or even theological reasons such as recognizing other persons as image-bearers or restoring shalom. But Paul appeals to none of those reasons in his letter to Galatians. His case for unity across ethnic lines is rooted in how Jesus fulfills/resolves Israel's story. Israel's sins are forgiven, the exile is over, God's Spirit has been poured out, and the ingathering of Gentiles has begun. If this is so, the functional segregation of much of the American church is not merely a political or cultural or social issue; it is a theological issue, and one that goes to the heart of the King Jesus Gospel.