The Bible is basically an instruction manual. We should also remember the Old Testament when living our lives, even tho' the New Testament (blood of Christ), covered alot of the way things used to be. I, personally, believe that Christ's main purpose was to teach us love for our fellow man/woman. He preached, (and taught), that thru LOVE we are to win people over, and nothing else. We are not to beat people over the head with the Word, but speak in terms of love and understanding. We are also to realize that , not everybody will accept. We are to "dust of our shoes ..." and move on, not hound people to death. I can tell by the general religious tone on AB, that most have run into the over-zealous, fire and brimstone Christians that sometimes end up doing more harm than good. But, we still should use the Old Testament as we study and try to gain a better understanding of the Bible. Just because the blood of Christ covers our sins, it doesn't mean it's okay to break the 10 commandments, for example.

The Old Testament is not "someone else's bible". It is the precursor of the New Testament. The whole thing points towards Jesus' coming. The OT shows how God established Israel specifically to provide a place, a nation and a family for the Messiah to be born into. Without the OT, the NT is like a plant without roots.

If you didn't have the Old Testament, you wouldn't have any insight into who God is. You'd be like, "Who the hell is this God character and why should I care about his love child who he sent to live with his mother?"

Mostly because of this verse... 2 Timothy 3:16 - KJV "All scripture is given by inspiration of God, and is profitable for doctrine, for reproof, for correction, for instruction in righteousness:" 2 Timothy 3:15-17 (in Context) 2 Timothy 3 (Whole Chapter)

Part of Jesus's inclusionary sayings specifically include the Old Testament, its Laws, and say they are still valid. “For truly, I say to you, till heaven and earth pass away, not an iota, not a dot, will pass the law until all is accomplished. Whoever then relaxes one of the least of these commandments and teaches men so, shall be called least in the kingdom of heaven; but he who does them and teaches them shall be called great in the kingdom of heaven.” (Matthew 5:18-19) This refers to Old Testament law, specifically Mosaic Law. "It is easier for Heaven and Earth to pass away than for the smallest part of the letter of the law to become invalid." (Luke 16:17) Same reference. "Do not think that I have come to abolish the law or the prophets. I have come not to abolish but to fulfill. Amen, I say to you, until heaven and earth pass away, not the smallest part or the smallest part of a letter will pass from the law, until all things have taken place." (Matthew 5:17) Not that this one is taken seriously by Christians. This says, literally, that until heaven and earth pass away, the Law is valid. Has heaven and earth yet passed away? "All scripture is inspired by God and is useful for teaching, for refutation, for correction, and for training in righteousness." (2 Timothy 3:16) "Did not Moses give you the law, and yet none of you keepeth the law." (John 7:19)

The OT was Christ's Bible. He was a Jew, and he quoted from it, and his teachings were a continuation of what was in the previous scripture. In much the same way the Last Testament (better known as the Holy Qur'an, HQ) has much that is sound and uncorrupted in the OT and NT. The central message of all three being the same - there is one God, He is the Creator of all, follow His guidance for success here and in the hereafter.

as most of the answers before have said it is because the OT is the background to the NT. If someone read the bible and it didn't have the OT that person wouldn't know who the heck God is. the OT also has things like the 10 commandments. If the OT wasn't there Christians wouldn't have any commandments to live by. The NT and the OT are the same. Even though different people wrote they were still talked to by God and told what to write down. So the OT and the NT are not different books they are only one book and that is the Bible.

then , What was God's PURPOSE of giving us the OLD? HE always has a PURPOSE... LEARN from OLD Testament(COVENANT) which had to CHANGE to bring in the NEW (COVENANT)Jesus....the SAVIOUR... we learn from OLD, LIVE by New.. Old Testament is PROPHECY FULLFILLED in the NEW TESTAMENT....by Christ...OT is SEED PLOT in GENESIS..the ALPHA AND OMEGA...