The Old Testament lays the foundation for the teachings and  events found in the New Testament. The Bible is a progressive revelation. If you  skip the first half of any good book and try to finish it, you will have a hard  time understanding the characters, the plot, and the ending. In the same way,  the New Testament is only completely understood when it is seen as a fulfillment  of the events, characters, laws, sacrificial system, covenants, and promises of  the Old Testament.

If we only had the New Testament, we would come to  the gospels and not know why the Jews were looking for a Messiah (a Savior  King). Without the Old Testament, we would not understand why this Messiah was  coming (see Isaiah 53), and we would not have been able to identify Jesus of  Nazareth as the Messiah through the many detailed prophecies that were given  concerning Him, e.g., His birthplace (Micah 5:2);  His manner of death (Psalm 22, especially vv. 1, 7-8, 14-18; Psalm 69:21), His resurrection (Psalm 16:10), and many more details of His ministry (Isaiah 52:13-15, 9:2).

Without the Old  Testament, we would not understand the Jewish customs that are mentioned in  passing in the New Testament. We would not understand the perversions that the  Pharisees had made to God’s law as they added their traditions to it. We would  not understand why Jesus was so upset as He cleansed the temple courtyard. We  would not understand that we can make use of the same wisdom that Christ used in  His many replies to His adversaries.

The New Testament Gospels and the  Acts of the apostles record many of the fulfillments of prophecies that were  recorded hundreds of years earlier in the Old Testament. In the circumstances of  Jesus’ birth, life, miracles, death, and resurrection as found in the Gospels,  we find the fulfillment of the Old Testament prophecies that relate to the  Messiah’s first coming. It is these details that validate Jesus’ claim to be the  promised Christ. And even the prophecies in the New Testament (many of which are  in the book of Revelation) are built upon earlier prophecies found in Old  Testament books. These New Testament prophecies relate to events surrounding the  second coming of Christ. Roughly two out of three verses in Revelation are based  on or related to Old Testament verses.

Also, because the revelation in  Scripture is progressive, the New Testament brings into focus teachings that  were only alluded to in the Old Testament. The book of Hebrews describes how  Jesus is the true High Priest and how His one sacrifice replaces all of the  previous sacrifices, which were mere portrayals. The Old Testament gives the  Law, which has two parts: the commandments and the blessing/curse that comes  from obedience or disobedience to those commands. The New Testament clarifies  that God gave those commandments to show men their need of salvation; they were  never intended to be a means of salvation (Romans  3:19).

The Old Testament describes the sacrificial system God gave  the Israelites to temporarily cover their sins. The New Testament clarifies that  this system alluded to the sacrifice of Christ through whom alone salvation is  found (Acts 4:12; Hebrews 10:4-10). The  Old Testament saw paradise lost; the New Testament shows how paradise was  regained for mankind through the second Adam (Christ) and how it will one day be  restored. The Old Testament declares that man was separated from God through sin  (Genesis chapter 3), and the New Testament declares that man can now be restored  in his relationship to God (Romans chapters 3–6). The Old Testament predicted  the Messiah’s life. The Gospels primarily record Jesus’ life, and the Epistles  interpret His life and how we are to respond to all He has done.

Without  the Old Testament we would not understand the promises God will yet fulfill to  the Jewish nation. As a result, we would not properly see that the tribulation  period is a seven-year period in which He will specifically be working with the  Jewish nation who rejected His first coming but who will receive Him at His  second coming. We would not understand how Christ’s future 1000-year reign fits  in with His promises to the Jews, or how Gentiles will fit in. Nor would we see  how the end of the Bible ties up the loose ends that were unraveled in the  beginning of the Bible, restoring the paradise that God originally created this  world to be.

In summary, the Old Testament lays the foundation for, and  was meant to prepare the Israelites for, the coming of the Messiah who would  sacrifice Himself for the sins of the whole world (1 John 2:2).  The New Testament shares the life of Jesus Christ and then looks back on what He did and how we are to respond to His gift of eternal life and live our lives in  gratitude for all He has done for us (Romans 12). Both testaments reveal the  same holy, merciful, and righteous God who must condemn sin but who desires to  bring to Himself a fallen human race of sinners through the forgiveness only  possible through Christ’s atoning sacrifice. In both testaments, God reveals Himself to us and how we are to come to Him through Jesus Christ. In both  testaments, we find all we need for eternal life and godly living (2 Timothy  3:15-17).