What is the major difference between OLD and New Testament?

The Old Testament is the story of the earth from the beginning. Then there are stories many of us grew up hearing of the Patriarchs, the Judges, etc. There is also in the Old Testament where God made the Jews His chosen people. He gave them their Law. The Old Testament also has many prophecies about the coming Messiah, someone the Jews were anxious to see.
The New Testament is the fulfillment of the Law, the fulfillment of the prophecies foretold about Jesus. Therefore, the whole world is now under the New Testament, since Jesus Christ fulfilled the Law. Christians are not under the Mosaic Law.

So, I would say the major difference between the two Testaments is the fact that in one people were subject to the Law of Moses., and in the other, they are to follow Jesus.


Answer:

In the Old Testament, God's Promise of Salvation [the saving of the world and mankind from his sins and his god -- and death; "...in thee shall all families of the earth be blessed..." - Gen.12:3] and the establishment of the Kingdom of God on earth -- both through Jesus Christ -- was foretold.

In the New Testament... the fulfillment of these two Promises saw their beginnings -- through Jesus Christ.

It began with the shedding of His Divine Blood -- and shall conclude with the fulfillment of the last of the Old Testament prophecies [and Judgment Day], spoken of in both testaments.

The Faithful of the Old Testament never saw what the Faithful in the New Testament witnessed:

"...blessed are your eyes because they see, and your ears because they hear. For I tell you the Truth, many prophets and Righteous men [recorded in the Old Testament] longed to see what you see but did not see it, and to hear what your hear but did not hear it." (Matt.13:16-17 NIV)

The major difference between the two testaments is that the New bears witness to the fulfillment of the Promises of the Old.

"For I tell you that Christ has become a servant of the Jews on behalf of God's Truth, TO CONFIRM THE PROMISES MADE TO THE PATRIARCHS so that the Gentiles [all the familes of the earth] may glorify God for His Mercy..." (Rom.15:8-9 NIV).

"...'Those who were not told about Him will see, and those who have not heard will understand.'" (verse 21)

That's another major difference. The advent of Jesus Christ... and the subsequent inspiration of the "Spiritually enlightened" New Testament writings, has facilitated a greater Understanding of God's Truth among more people. Including God's Truth found in the Old Testament.

"Then He opened their minds so they could Understand the Scriptures [the Old Testament]." (Luke 24:45 NIV)

All people don't understand, yet... but when the fulfillment of all the Promises is done... all will.