I think you are a little confused. The "Christian" bible is formed in two parts.......The Old Testament and the New Testament. They are not meant to be read in isolation of each other; http://www.bible.ca/b-Why-OT-NT.htm
This quote sums it up quite nicely.............."The two portions of God's will are so closely related it can be said about them that the Old is the New concealed and the New is the Old revealed."
