Why are Christians not Jewish when the Old Testament is in the Christian Bible?
The Old Testament records God's dealings with His chosen people, the Israelites, and the promise He made to them to bless the world through them. The New Testament records what Christians believe is the fulfilment of that promise, the birth of Christ, His life and the building of His church. Those of Jewish faith regard the Old Testament as their scripture. Those of Christian faith see both the Old and New Testaments as scripture.
Testament means "covenant." Christian Bibles consist of the Old Testament and the New Testament. The Old Testament book is essentially the Jewish Bible, also called the Tenakh. The book of the New Testament consists of the writings that document the coming of the awaited messiah, Jesus Christ, and the development of His church.
Originally, the Old Testament was the covenant between God and Abraham, wherein God promised to bless all nations through Abraham's progeny. This promise, Christians believe, pointed forward towards Christ. The Old Testament lists the law, the ten commandments and many rules of observance, as well as history, psalms and wisdom. In the New Testament, Jesus says "Think not that I am come to destroy the law, or the prophets: I am not come to destroy, but to fulfil."
In Luke 24:44, Jesus said "... all things must be fulfilled, which were written in the law of Moses, and in the prophets, and in the psalms, concerning me." And "For this is my blood of the new testament, which is shed for many for the remission of sins." Accordingly, Christians believe that the entire Old Testament was preparing God's people for the gospel of Jesus Christ.
The fulfilment of the law was accomplished by Christ's death and resurrection.Believers consider themselves not under the Old Testament law, but that by grace with the help of the Holy Spirit they follow the Law of Christ (Galatians 6:2) which is ..."Thou shalt love the Lord thy God with all thy heart, and with all thy soul, and with all thy mind. This is the first and great commandment. And the second is like unto it, Thou shalt love thy neighbour as thyself. On these two commandments hang all the law and the prophets." (Matthew 22)