Paul’s summation of the Gospel in 1 Corinthians 15 encapsulates the Gospel in the death, burial and resurrection of Christ.
People have often wondered how OT believers were saved since they were around before Christ was killed, buried or resurrected. But we know the Gospel is everlasting and there’s only ever been one and it’s the same one Abraham heard.
The Gospel is centered on death. The Bible is one bloody book. The life is in the blood. Without the shedding of blood there is no remission of sin. The day you eat of the tree you will die.
The Gospel, whether before or after Christ, was about death. Sacrifices made this abundantly clear. Cain eventually figured it out while Abel got it right away.
Paul goes on to explain what Gospel living is like after Christ’s actual death, burial and resurrection. It’s no longer I who lives but Christ who lives in me. We are buried with Him and raised up with Him. Mortify the deeds of the flesh. Our old man is crucified behold all things are new, etc.
How do you know you believe the Gospel? The same way Abel, Abraham and Paul knew–death becomes vital to you. We know we’re doomed, death is our wages for our sin. The sooner you see death, the sooner you will run to a Savior, a Savior who also died and now gives eternal life.
If you know this it will be reflected in a life that puts self to death and comes alive to the life of Christ.