Gospel means good news. In scripture, it relates to salvation (Ro 1:16): how we may be reconciled to God (Ro 5:10), delivered from both the penalty and power of our sin. (Ga 1:4)
This everlasting gospel is revealed and enabled in the divine sacrifice, of which the Old Testament animal sacrifices are a type (Jn 1:29); through His substitutionary death on our behalf, Christ became the propitiation for our sins, and not for ours only, but also for the sins of the whole world. (1Jn 2:2) We know God is satisfied with Christ’s work because of the resurrection. (Ac 17:31)
But even with the facts of the gospel laid out in plain view, until God opens our eyes, and turns us from darkness to light (Ac 26:18), we remain blind, dead in our trespasses and sins. (Ep 2:1-3) Those who continue in doubt (He 4:1), or to pursue disobedience (1Jn 3:9), who don’t love God (1Co 16:22), who aren’t being transformed into His likeness, are like dead men walking — they haven’t believed this good news: they haven’t been reconciled to God or delivered from the power of sin. (1Jn 3:10)
When Christ came He preached this good news (Lk 20:1), but never once explicitly mentioned His death, burial or resurrection. A sinful woman found forgiveness of all of her sins in Christ through this gospel; overwhelmed in grateful tears, she loved Him intensely. (Lk 7:47)
Abraham believed the gospel when he took God at His word (Ro 4:3), that one of his descendants (Ga 3:16) would be in number as the stars, and God counted this in Abraham as perfect righteousness. (Ge 15:5-6)
King David believed this gospel, and found a place of perfect reconciliation and righteousness in God without working for it, a place where God would never again impute sin to him. (Ro 4:6-8)
This same gospel was also preached to unbelieving Israel in the wilderness, but it fell on deaf ears. (He 4:2) Perhaps, as it was then, even so it is now, that very few perceive the gospel, though most of the world has heard of the death, burial and resurrection of Christ, and is aware that He died for our sins.
We aren’t reconciled to God, or saved, by acknowledging a set of historical facts, or by asking God to save us and committing our lives to serve Him; this is mere religion. We can know Jesus died for the sins of the world, and that He rose again, and still not believe the gospel, the good news, such that we’re saved from the power of sin and death. (Ro 8:2)
There’s only one gospel, one version of this good news, that’s legitimate; anyone preaching any other gospel is accursed. (Ga 1:8) Christ can quicken us (Col 2:13), make us spiritually alive, and take care of our sin (1Jn_3:5), and only Christ can do this. We can’t add anything to this, or take anything away from it, and still have the gospel.
There is no ritual that enables and facilitates receiving this gospel. (Ga 6:15) Salvation comes through an actual miracle of faith, where God gives us supernatural assurance and trust that He’s taken care of our sin in Christ (1Th 1:5), fully and completely, causing us to enter into His rest (He 4:3), and begins transforming our hearts with the living Christ, giving us a new nature (Ez 36:26), His own nature (Col 1:27), enabling us to love and obey Him.
Salvation is the receiving of God Himself, the divine Person, as He is. (Jn 1:12) This is the new birth; it is the work of God, not springing from the human will (Jn 1:13), but God conceiving us through His Word. (Ja 1:18)
Very few believe the gospel and find this salvation (Mt 7:14); we should each labor to enter into this rest (He 4:11), striving to enter (Lk 13:24), diligently ensuring our own calling and election. (2Pe 1:10)