I assume that you are referring to the Old and New Testaments and I will answer your question with this thought. The two great divisions of the Bible are the Old Testament and the New Testament. In order to help you understand them better, I will compare them in several ways.
AS TO ORDER:
The Old Testament comes first and is followed by the New Testament.
AS TO SIZE:
In length alone, the Old Testament is approximately 3 times as large as the New Testament. That is, the Old Testament makes up about three-fourth's of the entire Bible and the New Testament makes up about one-fourth of the entire Bible. The Old Testament is made up of 39 books containing a total of 929 chapters. The New Testament is made up of 27 books containing a total of 260 chapters.
AS TO TIME WRITTEN:
The Old Testament is much older and was written over a longer period of time. The Old Testament was probably written between 1500BC and 400BC, a time of over 1,000 years. The New Testament was probably written between 40AD and 100AD, a time capable of being spanned by one person's lifetime.
The two Testaments are also placed before and after a crucial event in the history of mankind. The Old Testament was all written before the coming of Jesus Christ. The New Testament was written after Jesus had already come.
AS TO AUDIENCE AND PURPOSE:
Most of the Old Testament was written to the Jewish people and was written to instruct and guide the Jewish people in their obedience to the law and their life as a nation especially chosen by God. The New Testament is written to present Jesus Christ to all people. However, much of it is written to believers of Christ and to the churches as instruction and guidance in the Christian life.
AS TO TESTAMENTS:
The names of these two divisions are very important. They are both called testaments. A covenant is a special kind of covenant or agreement. A testament is an agreement about what will happen when someone or something dies. It is related to the idea of a last will and testament. Hebrews 9:16-17 refers to this: “For where a testament is, there must also of necessity be the death of the testator. For a testament is of force after men are dead: otherwise it is of no strength at all while the testator liveth.”
The Old Testament is based on the sacrificial death of animals and is dependent on these sacrifices for the temporary forgiveness of sins. The New Testament is based on the sacrificial death of Jesus Christ for our sins. Through His shed blood, Jesus became “the mediator of the new testament” (Hebrews 9:15).
AS TO CONNECTION:
With all these differences, you may wonder why Christians accept both the Old and New Testaments as scripture. We believe that they are both of God and are both absolutely true. God did choose Israel as a special nation. Through the sacrifices and ceremonies given to them, He shows us much about Himself. In the teaching and preaching of the Old Testament, the human heart is revealed for what it is.
However, it is in the New Testament that the Old is fulfilled. The types and pictures of the Old Testament point to the coming of Jesus Christ and His offer of Himself as the Saviour of the world. That which is in the Old concealed is clearly in the New revealed. The two go together in complete harmony. The New completes the Old. They are part of the absolute plan of God.