The gospel is that Christ died for our sins, according to the Scripture, that He was buried, and that He was raised from the dead (1 Cor. 15:3–4). The heart of the gospel is Jesus Christ Himself—the person and work of Christ, the sin-bearing work of Christ, as well as the perfection of His life that is imputed to those who believe. Out of the Reformation came the great solas and the three in the middle, sola fide, sola gratia, and solus Christus are the heart of the gospel. In other words, salvation is by grace alone, through faith alone, in Christ alone. That is the synopsis, in its most succinct form, of what the gospel is.
The word gospel, euangelion, means “good news,” which presupposes that there is also bad news. There is no good news unless you first know the bad news. The book of Romans begins with bad news (Rom. 1:18). The gospel does include a message about sin, which is any want of conformity to the law of God. Sin is a transgression. The Bible says, “The wages of sin is death” (Rom. 6:23), and “The soul that sins, it shall surely die” (Ezek. 18:20). There must be the understanding in the gospel that I am under divine judgment; my sins have offended a holy God; I am separated from God; there is nothing I can do, in and of myself, to remove the pollution and the condemnation of that sin; and my only hope is found in Jesus Christ, who died upon the cross for my sins. That is the heart of the saving message of the gospel.
We could extend out and add other aspects, such as the virgin birth of Christ, His sinless life, His substitutionary death, His bodily resurrection, His present intercession in heaven, His soon return, and so on. All of that is interwoven as to which Christ we call upon and believe.
Galatians 1:9 says, “If any man preach another gospel...” There is “another gospel” that is corrupted by the addition of human works to my salvation. For example, the Roman Catholic Church teaches another gospel. It is a damnable gospel. It is a bridge that doesn’t get you to the other side. It is to lay hold of a log that will not save you as you’re drowning. There is only one true saving gospel, and it is by grace alone, through faith alone, in Christ alone. Rome would say “grace and…;” “faith and…;” “Christ and...” The Reformers, true to Scripture, took out the word “and” and added the word “alone” or “sola.” That frames the essence of the gospel of Christ. Any addition of human works, merit, goodness, or religiosity is a corruption of the gospel. Salvation is not a reward for the righteous; it’s a gift for the guilty.
The clarity of the Christ whom we preach is critically important. Isaiah 53 is one of the clearest presentations of the substitutionary death of Christ, which is at the very heart of the gospel of Christ. So, that is the gospel in summary form, and the five doctrines of grace bring more clearly into focus the saving work of God in the gospel.