9 Q. Could you give a more definitive explanation of what the gospel is?
A. The gospel, or the good news that delights the heart of the poor condemned sinner, is a revelation of the fatherly and immutable will of God, in which He promised us, who are unworthy, that all our sins have been washed away and pardoned not just for the rest of our lives but, indeed, forever. He carries out this promise by giving His Son to die for us and by raising Him. Since Christ died not in His own sin but in ours (as if He Himself had committed it) and arose out of this same sin as a mighty victor (1 Cor. 15:17), it follows that there is not a single sin of ours for which He has not paid. If even one of all those sins that Christ took upon Himself had not been paid for, He would have had to remain in the tomb and could not have risen. For where there is even one sin, there is also eternal death, as God Himself says in Deuteronomy 27. Also, “The wages of sin is death” (Rom. 6[:23]). Since, therefore, Christ arises out of all our sin as a victor in our flesh (which He assumed and forever retains), this is public testimony to us that we are considered as pure and righteous in the eyes of God as Christ Jesus was when He arose from the grave (Rom. 4; 1 Cor. 15:17). Along with that, God through Christ both promises us in the gospel and then actually gives us the Holy Spirit. The Spirit turns our hearts from sin and from the kingdom of the Devil to Himself, bears witness that we are children of God, initiates in us both joy in God and eternal life here below, and brings it to completion in us up in heaven. All of this God freely offers and gives to us in the gospel, without any regard to our past, present, or future merit or piety. He applies it to us by grace through faith, so that whoever boasts, boasts in the Lord (Jer. 9[:24]; 1 Cor. 1[:31]).
It can also be described more briefly: the gospel is a revelation of the fatherly and immutable will of God, in which He promises all believers (1) that their sins have been pardoned from eternity and shall be forever forgotten, and (2) that He will freely give them the Holy Spirit and eternal life, without any past, present, or future merit of ours, because of the voluntary sacrifice of this most exalted person, Christ—truly God and truly human. This sacrifice was there before the face of God from eternity, then was promised, and now has been carried out and completed, retaining forever its efficacy for our full redemption (Eph. 1).
Caspar Olevianus, A Firm Foundation: An Aid to Interpreting the Heidelberg Catechism, trans. Lyle D. Bierma, Texts and Studies in Reformation and Post-Reformation Thought (Carlisle, United Kingdom; Grand Rapids, MI: Paternoster Press; Baker Books, 1995), 8–9.