The gospel, in general, is this: the good news of what God has done in and through Jesus Christ to reconcile sinners to himself. More largely it is this: as all of mankind has fallen in sin through their first parent, Adam, and are unable by their own works and wisdom to restore themselves to God, God in grace has sent his one and only Son to fulfill God’s law, take the punishment of sin upon himself, and mediate for his people.
The eternal Son of God was “born of woman, born under the law, to redeem those who were under the law, so that we might receive adoption as sons” (Gal. 4:4-5). He fulfilled the law in every respect, being without sin (Heb. 4:15). As the Holy and Righteous One (Acts 3:14) he substituted himself in the place of sinners, bearing their sin, guilt, shame, and punishment on the cross. As the Scripture says, “For our sake he [God the Father] made him [Christ Jesus] to be sin who knew no sin, so that in him we might become the righteousness of God” (2 Cor. 5:21). Again, “Christ redeemed us from the curse of the law by becoming a curse for us–for it is written, ‘Cursed is everyone who is hanged on a tree'” (Gal. 3:13). After being crucified, three days later Jesus was resurrected from the grave, defeating the curse of sin, which is death (Acts 2:24; 1 Cor. 15:54-56). Having provided the perfect sacrifice and risen from the grave in victory, Christ now sits at the right hand of God where he intercedes for all his people (Rom. 8:34).
Accompanying this good news is the call to repent and believe; turn from your sinful ways and entrust your soul to him who is both Judge and Savior (Acts 10:42-43). “For the wages of sin is death, but the free gift of God is eternal life in Christ Jesus our Lord” (Rom. 6:23).
For a fuller explanation of the gospel, we recommend the book, What Is the Gospel?, by Greg Gilbert.