Complicated answers and I am not an expert. Here is some of what I've learned. Because following the Covenants(promises) along the way thru the Old Testament. We see that bad things happen to those who break the covenants with God. When they turn to evil. They knew they were disobeying God. The Jews followed the Word of God for centuries. Waiting for the promised messiah. They today do not believe Jesus is that promised saviour. Christians see that Jesus is the Son of God and the fulfillment of that promise from right after Adam and Eve are thrown out of Eden. As early Christians were Jews converted by their belief that Jesus was the Messiah and many traditions and beliefs match the practices of the Jews. It makes sense those writings would be important. After all Jesus was raised a Jew. Jesus says. I have come not to abolish the law but to fulfill it. Big changes began to occur within His small group of followers. The concept of the Trinity is another part to look at. God the Father, God the Son, God the Holy Spirit. All 3= God.