I'm just wondering about the Bible with the Old Testament and New. Most Christians prefer to use the New Testament (especially the rules).
Some of them don't follow the rules in the Old Testament.But why?
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Not to be picayune here, but the Old Testament is not one book. It is 39. Likewise, the New Testament is 27. This question is akin to asking "Which is more important - the Shakespearian corpus or writings of Camus, Satre, Locke, Asimov, Vonnegaut, C.S. Lewis, and Orson Scott Card. You are asking to compare two libraries from different historical eras with different purposes in writing.
In a nutshell, the Old Testament is a collection of works from about 1000 BC to 400 BC, chronicling the story of the Jews - God's chosen people, to whom he chose to love, redeem, and instruct. It includes:
The New Testament contains the story of Jesus, who was born a Jew, and the people who knew Him. Specifically, there are:
After that, there are 23 other books, including:
It was written between about 50 AD and 100AD, and focuses on the idea that God became a man, died, was buried, rose again, and through that act of unmerited favor, saved mankind from sin.
Finally, what you are asking - to pick "the most important" violates the principle that all Scripture is fully important, and to pick a 'canon' within a canon is considered heresy. Not that Christians don't regularly do that, but technically, its considered wrong.
To answer the question very crassly, given a choice, most Christians would, if forced to choose, say "the New Testament," because it focuses on the saving power of Jesus. This is evidenced by the fact that Christians will sometimes carry "abbreviated bibles" with just the New Testament and the Psalms.
Then again, there are lots of people who will only eat their meat and dessert, and not the refreshing goodness of bread and vegetables either. To live on just the NT is to deny yourself the background and the full story of God's redemptive purpose. For Christians, it is all profitable and all important.
Christianity is incomplete without both the Old Testament and the New Testament, so neither one is more important than the other. They are intertwined. As one person put it,
The Old Testament records Creation, God's interaction with mankind, mankind's failure in the age of innocence, conscience, and law. It reveals that, just as Adam's attempts to cover over the shame of his own sin was futile, so all our efforts to atone for our own sin are futile. As mankind is helpless, God points to His own provision to come.
The New Testaments is the fulfillment of this promise of God. He promised He would come, and He did. He promised He would atone for sin, and He did. He promised a new covenant, and He introduced one.
Only the Old and the New Testaments together make sense. The Old without the New tells of a Messiah that never comes. The New without the Old tells of a Messiah that came, but with no indication why He did. Furthermore, the New Testament references the Old quite often, which is also testimony to the fact that the two are inseparable.
Regarding the "rules" of the Old Testament, those were introduced by Moses as the Mosaic Covenant around 1500 B.C. and were in effect until the New Covenant was revealed with the coming of Jesus Christ. The Old Testament spans much more than 1500 years, so these laws were in effect for just a fraction of that time. The Mosaic Covenant was a specific covenant for a specific time with a specific people--from 1500 B. C. to the time of Christ and with the Jewish people. So, while God still commands us to live holy lives, we, as people living in the New Covenant, are not bound to the ceremonial laws of the Old Covenant.
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