The key to understanding the Old Testament is to realize that the entire Bible is an account of God's progressive revelation. Just like with any book if you miss the first part and attempt to finish it; you will have a hard time understanding the characters, the storyline, and the ending - the Bible is not any different. This is one reason understanding the Old Testament is so important.  The Old Testament deals mainly with the relationship between God and the nation of Israel. Throughout the Old Testament a foreshadowing of the coming Messiah, Jesus Christ, is seen in types, promises, and prophesies. 

In the New Testament the long awaited Messiah, Jesus Christ, is manifested.  The New Testament can be fully understood only when it is seen as being built upon the foundation of the Old Testament and its events, people, laws, sacrificial system, covenants, promises, and prophesies. If we had just the New Testament, we would come to the gospels and not know why the Jews were looking for the Messiah to be their King. In Old Testament passages such as Isaiah 53 we learn why Messiah came. Without the Old Testaments detailed prophesies we would not have been able to recognize Jesus of Nazareth as the Messiah. In Micah 5:2 we learn of His place of birth and in Psalm 22 and 69 we the way He would die.  Psalm 16:10 speaks of His resurrection and throughout the Old Testament we see numerous other particulars of His life.

We would not comprehend the Jewish customs that are mentioned in the New Testament without the Old Testament. Nor would we understand how all of the Jewish feasts and festivals point directly to Jesus Christ.  We would not realize the twistings of and additions to the law the religious leaders had made.  For these reasons and many more, it is important that all followers of Jesus Christ understand the Old Testament.