What is the difference between the Old and New Testament?

Why do Christians only follow the New Testament?

The Old Testament mainly deals with the stories of the prophets sent to the children of Israel i.e. the Jews before Jesus (alayhi salaam) while the New Testament focuses on the story of Jesus and the events that occurred after his departure from this world.

The Jews accept the Old Testament but reject the New Testament since they do not regard Jesus to be a messenger of God. However the Christians are supposed to believe in both the Testaments. It’s not true that Christians only follow New Testament. Some Christians may say that, but majority do accept both. The Christians believe in the complete bible as the word of God which comprises of both the Old and the New Testaments. They may not follow the laws and the commandments mentioned in the first 5 books of Moses (alayhi salaam) since Paul has nailed the law on the cross but they accept the creation story, other stories of the prophets, prophecies regarding the coming of Jesus, etc. mentioned in the Old Testament.

The complete Bible, i.e., the Old Testament and the New Testament put together, contains 73 books. However, the Protestant Bible i.e., the King James Version, contains only 66 books as they consider 7 books of the Old Testament to be Apocrypha, i.e., of doubtful authority.

Therefore the Old Testament of the Catholics contains 46 books and that of the Protestants, 39 books. However the New Testament of both these sects contains 27 books.