Here are two simple reasons why I believe Christians should read the Old Testament:-
1. It was the Scriptures that Jesus, the apostles, and all the New Testament authors used
The authors of the New Testament never imagined that they were writing something 'new' that was disconnected from or 'replaced' the old. Nowhere in either the 'Old' or 'New' testament is there mention of one negating the other or of the other being irrelevant. In fact, Jesus' words in Matthew 5:17 clearly indicate that He had not come to abolish the law but to fulfill it. The early believers viewed the Scriptures as the foundation of God's plan. They articulated the beauty, majesty, and person of Jesus through the verses and lens of the Old Testament. That was the entire 'Bible' they ever had until the 'New Testament' written in the 1st Century AD. Even Jesus taught in a synagogue in Nazareth from the book of Isaiah (Luke 4:16-21). He even expounded on the 'Old Testament' regarding Himself to the two disciples on the road to Emmaus; "Then Jesus took them through the writings of Moses and all the prophets, explaining from all the Scriptures the things concerning himself." (Luke 24:27)
2. The Old and New Testament is one unified story; God's story.
The Old Testament sets the stage or the backdrop to the New Testament, in God's grand story line. Numerous times in the New Testament are Old Testament Scripture used and referenced to. Crossway gives a figure of 263 times a New Testament book cites and Old Testament Scripture. If clear allusions are taken into consideration, the figure is much higher, up to more than 10% of the New Testament is made up of citations or direct allusions to the Old Testament. This fact alone should give us motivation to look back to what happened in the past before the New Testament was written. The Old Testament gives us context and background into correctly understanding and interpreting the New Testament. Just imagine reading the chronology in Matthew 1 and not having any understanding at all of anyone mentioned in the genealogy of Jesus! If we do not understand the Old Testament, we will have trouble understanding the New; especially in the area of messianic prophesies concerning Jesus who is essentially; the 'Gospel of the Kingdom' that He himself proclaimed (Matt 4:23).
In conclusion, it is clear that ALL Scripture is breathed out by God and is profitable for teaching, for reproof, for correction and for training in righteousness (2 Timothy 3:16-17). Paul wrote that verse referencing the Old Testament! Oh, that our heart would burn within us as we read the Old Testament Scriptures just like it did for the two disciples did when Jesus expounded it to them (Luke 24:25-32).
References:
http://www.bible-researcher.com/nicole.html
http://www.theopedia.com/New_Testament_use_of_the_Old_Testament
