The whole Bible is about Jesus Christ and his Gospel. In the Old Testament we see God bringing forth a people and a nation from whom the savior of the world would come. God chose Patriarchs, prophets and others to reveal his plan of salvation. The Old Testament saints were saved and justified by faith in God's promise of a savior.
"Abraham believed God and it was counted unto him for righteousness" Romans 4:3.
There is a thread that runs through the Old Testament concerning God's promise of a savior. Many of the Jews looked forward to seeing God's promise of a savior, but died without ever seeing it become a reality, Hebrews 11:39. They were saved and justified by faith. They looked forward to seeing the savior. We look back. It is much easier to look back than it is to look forward. We have the whole revelation of Jesus Christ and his Gospel in the New Testament.
God's promise of a savior becomes a reality in the New Testament books of Matthew, Mark, Luke and John.
"For unto you is born this day in the city of David a savior, which is Christ the Lord" Luke 2:11.
The long awaited savior finally arrives, but not many knew who he was, nor did they know what his purpose was, John 1:12. The apostles knew who he was, 1 John 1:1-4. But even some of them did not know what his purpose was until the day of Pentecost. Acts chapter 2 is the big reveal. For the very first time the Gospel comes into the world in the power of the Holy Spirit. The lowly disciples are transformed into powerful preachers of the Gospel, Acts 4:18-20. Pentecost gave birth to the New Testament church. Nothing is the same after Pentecost. The apostles went everywhere preaching the Gospel of Jesus Christ. Thousands and thousands of Jews are born again by the word of God (the Gospel) 1 Peter 1:23 and become witnesses for Christ and his Gospel.
WHAT IS THE GOSPEL?
The Gospel is that work that Jesus did in our name and on our behalf that justifies the ungodly, Romans 4:5 and reconciles us and the world unto God, 2 Corinthians 5:18, 19. In the Gospel Jesus comes into the world as God's new Adam and our new humanity, 2 Corinthians 5:17. His mission is to do for us that which we cannot do for ourselves. By his righteous sinless life he offers to God the Father a life of perfect obedience according to his Holy Law. Jesus is the end and the fulfillment of God's law, Romans 10:4.
That was not enough. Something had to be done about our sins and the sins of the whole world,
"For he has made him to be sin for us, who knew no sin; that we might be made the righteousness of God in him" 2 Corinthians 5:21.
In Jesus Christ we have been justified, sanctified and redeemed, 1 Corinthians 1:30. We now stand before God as holy and complete in Jesus Christ, Colossians 2:10. If the Bible is read in the light of this historical Gospel of Jesus Christ it will make sense. If you try to add your religion to the Bible, it will not make sense.