The Bible is a Book that has two main divisions in - the Old Testament and the New Testament. Even though there are these distinct parts of the Bible - it still has only one theme - redemption. This is necessary to understand because the whole Bible revolves around this theme, and some have said that there is a scarlet thread (of blood) that runs form cover to cover.
Before the differences are looked at, let's consider some other similarities that exist between the two halves. The books were all written by men - even the two books that bear the names of women - Ruth and Esther. The theme of both Books is redemption, and in both Books redemption occurs through faith. The Redeemer is the same in the Old Testament and the New - though it was not as clear in the Old. God always had the plan of sending His Son to become the Redeemer by dying for the human race (see Isaiah 53, and Psalm 22). Also, in both the Old Testaments and the New, the blood of sacrifice is necessary for redemption (Leviticus 17:10 and Hebrews 9:22)
Another similarity is that the Bible begins with a tree, the Tree of Life - from which mankind is forbidden to partake after they sinned, and the Bible ends with the Tree of Life - offered freely to the redeemed to partake of its fruits. It also begins with man and God walking together in fellowship in the beginning, and once again, after God's working to redeem man, in the end, forever.
The Main Difference Between The Two
All of the differences can be summed up by saying that the Old Testament begins the teaching, or revelation, while the New Testament finishes (or completes) that same revelation. In other words, the story is incomplete without the New Testament. The completion that is accomplished here is not just referring to a completed Book, but something more. The complete revelation of God to man, and of God's finished work of man's redemption, is completed in Christ - Who is God in the flesh. The New Testament is God's infallible record of what Christ has done - and will yet do.
Other Differences Between the Two
1. The Old Testament deals largely in pictures and types.
Much of the truth about Christ that is in the Old Testament was hidden or unknown until Christ came. While they did know that everything relating to the Tabernacle and the Temple was to be made "according to the pattern," most of them probably did not understand that the Pattern referred to was Christ Himself. The New Testament, however, reveals what many of the things in the Old Testament meant.
A couple of the events of the Old Testament were prophetic through their illustrated truth, and of which the New Testament tells us that they were types (pictures) of Christ:
Abraham's offering of Isaac - pictured the Father Who willingly offered His only begotten Son; and of the Son Who refused not to die – both believing that God would raise him up – which is exactly what happened in Christ’s case
Moses lifting up the serpent in the wilderness - pictured Christ being lifted up for us, and putting Satan and his power to death.
Moses striking the Rock - he was to do it once because it pictured that Christ would be smitten once, and from Him would flow living waters.
2. In the Old Testament, once the Temple was built, God dwelt in the Temple - distant (the place of His revealed presence), but in the New Testament, God lives in the heart of each believer - close and personal, showing that the fellowship with God has been restored.
3. The Old Testament begins with God calling out a people-nation to Himself (Genesis 12), while the New Testament shows Him beginning with that nation, through 12 Jewish apostles, and calling unto Himself the whole world.
4. The Old Testament largely deals with the Law of God, but in the New Testament, the focus is on the grace of God. This is not because God had changed in any way, but it was because the complete Sacrifice for the forgiveness of sins had been accomplished - Christ died for our sins.
5. The Old Testament shows God remaining largely hidden, and access to Him could only be gained through the Temple, and with an animal sacrifice. The New Testament shows that Christ has given every believer free access to the Throne of God.
The difference between the two is further illustrated in the Bible by an incident that occurred just after Christ died on the Cross. In the Temple, there was a thick curtain, behind which only the High Priest could enter once a year. When Christ died, this curtain was ripped from the top to the bottom without human hands. This miracle showed to all who saw it that God would no longer remain hidden away behind these pictures, but now every man could have full access to God through Christ by faith.