"New Testament? Old Testament? What is the difference, and why does it matter?" Understanding the difference between the Old and New Testaments is one of the most important foundations that must be laid to properly understand God’s Word. A brief summary is provided here of the conclusions from a deeper study that is based upon Bible verses.
The word "Testament" can have different meanings depending on the context, so the first thing to do is to define the word "Testament" as the Bible defines it. Some religious organizations wish to use one meaning of the word that makes it the testimony of a witness. Therefore, the Old and New Testaments would be older and newer testimonies about God and His people. Adopting this meaning conveniently opens the door for them to usher in an even newer testament, another revelation from God. However, the Bible teaches that it is already final, complete, and nothing will ever be added to it.
So how does the Bible use the word? The Bible uses the another meaning of the word "testament". It is used in the sense of "last will and testament", or as what we just commonly call a "will". The testaments are a type of will, an issue of instructions to be carried out once the one giving the will has died. Therefore, the New Testament is Christ’s last will and testament for us to keep now that he has left the earth. However, the word "testament" is one of multiple descriptive words used by the Bible. It served as an illustration to explain the bounds of the covenants and one of the reasons that necessitated Christ’s death. But the word more often used in the Scriptures is the word "covenant". This word describes another facet of these testaments. They are not just a last will and testament, but rather a divine agreement extended by God. The New Testament, or Covenant, is a promise from God that He will save us if we obey His Testament.
"So what's the difference? Why have an Old and New Covenant?" Both covenants were ushered in by “the shedding of blood” - blood of animal sacrifices for the Old, and the blood of Christ for the New. But, Christ’s blood of the New Covenant does what could not be done in the Old - forgive sins. The Old Testament prophets hundreds of years before, foretold the coming of a second covenant. Moreover, the New Testament also teaches that Old Testament was imperfect because it could not offer forgiveness of sins. Instead, each year the sins were "rolled back" until Christ’s ultimate sacrifice. The entire book of Hebrews is devoted to showing that the Old Covenant is inferior to the New, and that New Testament Christians should not abide by this inferior covenant when they have something so superior. In fact, the writer of Hebrews actually quotes the a passage from Jeremiah to make this point (Hebrews 8:6-13).
Another significant difference is who God gave the covenant to. The Old Testament was a covenant given strictly for the Jews, or Israelites. Non-Jews, or Gentiles, could adopt the Jewish covenant and become "prostelytes", but there was no special law and relationship offered that was for the Gentiles as the Old Testament was for the Jews. However, the New Testament made no distinction. One of the great things about the New Testament is that God’s message and special covenant relationship is extended to all races and peoples. Consequently, the writings of the Old Testament generally track the story of the Israelites with a few exceptions, and the New Testament writings contain the gospel of Jesus and of the spread of the gospel all over the world, as well as directions for the new Christians.
So what is the application for us today? There are two things we should learn and put in practice. One, the Old and New Testaments are synonyms for Old and New Covenant, not testimonies. Second, the Old Testament was done away at the cross by Christ’s death, which ushered in the New Testament. The third and final lesson is that the Old Testament has been done away as an authority for how to be saved and how God’s church should operate. However, the Old Testament is not been made worthless, but it serves as a great source of examples that are necessary for our spiritual growth and health (I Corinthians 10:6, 11-12 and Romans 15:4).
Therefore, let us always bear this distinction in mind. We must be diligent to use the New Testament as authority, and to use the Old Testament examples in their proper place. If we do this, we will solve many of the disagreements that separate those who would worship God. With this fundamental pillar in place, we are now ready to examine the actual process of establishing Biblical authority, "doing all in the name of the Lord".
To read an exchange and more discussion on this topic, please visit this thread and this thread on our forums.
Examine more in depth study on this topic, complete with Bible verses.