"For I delivered to you as of first importance what I also received, that Christ died for our sins according to the Scriptures, and that He was buried, and that He was raised on the third day according to the Scriptures"
- 1 Corinthians 15:3-4
Right there, that right there, is the Gospel in its most simple and concise form. That is what someone must believe to be saved.
"He made Him who knew no sin to be sin on our behalf, so that we might become the righteousness of God in Him."
- 2 Corinthians 5:21
"for all have sinned and fall short of the glory of God, being justified as a gift by His grace through the redemption which is in Christ Jesus; whom God displayed publicly as a propitiation in His blood through faith. This was to demonstrate His righteousness, because in the forbearance of God He passed over the sins previously committed; for the demonstration, I say, of His righteousness at the present time, so that He would be just and the justifier of the one who has faith in Jesus."
- Romans 3:23-26
You want the summation of it?
Mankind is wicked by nature and choice and is under the just condemnation of God (Romans 1:18-21, Psalm 53:2-3), who is infinitely good and holy. But God is love and desired to save sinners (John 3:16).
God the Son became a man in the person of Jesus Christ (Colossians 2:9), born of a virgin (Matthew 1:20-21) under the law (Matthew 5:17). He fulfilled the law perfectly without sin (Hebrews 4:15).
He was rejected by men and nailed to a cross to die. Upon that cross God the Father displayed him publicly as a propitiation (wrath-appeasing sacrifice) for the sins of the world, especially of those who believe (1 John 2:2, 2 Timothy 4:10). He was buried and on the third day rose again (1 Corinthians 15:3-4).
Now God commands all men everywhere to repent (Acts 17:30), and salvation comes by repentance toward God and faith in the Lord Jesus Christ (Acts 20:21). To repent is to confess and forsake your sinful way (Isaiah 55:6-7), and to believe in Christ is to trust in him alone for salvation (Galatians 3:1-2).
"and He said to them, Thus it is written, that the Christ would suffer and rise again from the dead the third day, and that repentance for forgiveness of sins would be proclaimed in His name to all the nations, beginning from Jerusalem."
- Luke 24:46-47
Upon conversion one is grafted into the body of Christ and becomes a partaker of all spiritual blessings in Christ (Ephesians 1:3-14) and nothing shall be able to separate the believer from the love of God in Christ Jesus (Romans 8:38-39, John 6:38-39, John 10:27-28, 1 Peter 1:3-5).
This my friend is the Gospel, the good news, the really really really good news, that we are to proclaim to the whole world (Mark 16:15).