The gospel of Christ is the good tidings that God has revealed concerning Christ. As all mankind was lost in Adam and became the children of wrath, put under the sentence of death, God, though He left His fallen angels and has reserved them in the chains of eternal darkness, yet He has thought upon the children of men and has provided a way of atonement to reconcile them to Himself again.
The second Person in the Trinity takes man’s nature upon Himself, and becomes the Head of a second covenant, standing charged with sin. He answers for it by suffering what the law and divine justice required, and by making satisfaction for keeping the law perfectly. This satisfaction and righteousness He tenders up to the Father as a sweet savor of rest for the souls that are given to Him.
And now this mediation of Christ is, by the appointment of the Father, preached to the children of men, of whatever nation or rank, freely offering this atonement unto sinners for atonement, requiring them to believe in Him and, upon believing, promising not only a discharge of all their former sins, but that they shall not enter into condemnation, that none of their sins or unworthiness shall ever hinder the peace of God with them, but that they shall through Him be received into the number of those who shall have the image of God again to be renewed unto them, and that they shall be kept by the power of God through faith unto salvation.
That these souls and bodies shall be raised to that height of glory that such creatures are capable of, that they shall live forever enjoying the presence of God and Christ, in the fullness of all good, is the gospel of Christ. This is the sum of the gospel that is preached unto sinners.
From Gospel Conversation (c.1657) and reprinted by Ligonier Ministries in 2006.