The four gospels of Matthew, Mark, Luke and John are the foundation of the New Testament and form a bridge between the law of the Old Testament and the grace of the New Testament. The Old Testament cannot be properly understood without the revelation of Jesus that the New Testament gives, for all of the law and the prophets pointed to Jesus (1 Pet. 1:10-12). The symbolism of the tabernacle and sacrifices, along with the ordinances of the law, were all shadows or pictures of Christ (Col. 2:16-17)
Likewise, the New Testament cannot be fully appreciated without an adequate understanding of the Old Testament. A person without the knowledge of God's judgement on the sin of adultery, as revealed in Exodus 20:14, Leviticus 18:20 and 20:10, would not realize the extent of God's love and forgiveness as revealed in John 8:3-11.
Therefore, it is fitting that the combination of the Old and New Covenants comprise God's Word to us, but as Christians, it is imperative that we realize the difference between the two and live exclusively under the New Covenant as revealed in the 26 books of the New Testament (Gal. 3:11-14; 5:1; Heb. 7:18-19; 8:7-13).
The gospels reveal Christ unto us. Certainly not everything that Jesus did is revealed (Jn. 20:30-31; 21:25), but the accounts that the gospel writers give reveal the very nature and person of Jesus. Even with all of the great doctrinal truths revealed in the epistles, our total picture of God would not be complete without the accounts of Jesus' life and ministry.
Jesus is the express image of God's person (Heb. 1:3). Jesus said of Himself, "he that hath seen me hath seen the Father" (Jn. 14:9). So, in recording the events of Jesus' life, the gospel writers teach by precept what the epistles teach through doctrine. The gospels are written down from a viewpoint that refrains from interpretation and expounding doctrine, but rather lets the life and actions of Jesus speak for themselves.
These four gospels have stood the test of time and have come to us as the inspired scriptural account of Jesus' life and ministry.
The need for four gospels became apparent upon studying them. Each gospel was written with a specific purpose in mind for a specific group of people. Therefore, taken as a whole, they present a complete picture of Jesus to all men. To combine all the events and different presentations of the events into one gospel would make such a voluminous work that the overall story would be lost, and many would not go to the effort of studying out these truths. Also, the revelation of Jesus Christ was of such magnitude that no one man, even inspired of the Holy Ghost, could present it completely.