Ever since the early church included the gospel of John in the New Testament canon, there has been no shortage of suggestions as to the purpose of writing the fourth Gospel. Some claim the purpose is to supplant or replace Matthew, Mark, and Luke, while others insist the fourth gospel is to supplement or interpret them. Still others believe the author was largely unaware of the other gospels or chose to be independent of them. While it is possible to present the differences between the four Gospels so as to suggest this gospel is trying to supplant the others, careful analysis of the gospels reveals a high degree of interdependence. While the fourth gospel is different, the gospels share considerable unity, which makes sense since all four books focus on the works and teachings of Jesus.
Some suggest the purpose of the gospel is to attack Judaism, but reputable scholars universally reject this suggestion. Others believe it is to counter the popularity of John the Baptist. Another idea is that it is written for a Greek reader because of references to “logos” in the prologue, the dualism found throughout the Gospel and Hellenistic concepts. However, research reveals the presence of Semitic and Aramaic tendencies consistent with Judaic thought of the first century Christian Era (CE) debunking this idea. Others opine that the gospel seeks to counter the influence of Gnosticism. The widely held dogma that the fourth gospel was written in the second century is the lynch pin for this suggestion, but the discovery of Papyrus 52 dating the writing of the gospel to before 100 CE significantly devalues the Gnostic threat as the reason for the fourth gospel. The Gnostic heresy would not become a serious threat until the second century and beyond.
Others go so far as to suggest this gospel is Gnostic, but it is easy to refute this idea when one examines what the Gospel and Gnosticism teach about the creation of the world and the relationship between God and the world. The two are spectacularly different. “In Gnosticism God and world are dualistic opposites. In the Johannine prologue all things – the ‘heavens and the earth’ of Genesis 1:1 – have been made by the Word that was with God and was God.”  John Ashton notes that the dualism in the fourth gospel is not like that of the Gnostics: cosmological in view but rather moral. Ashton’s understanding of John’s dualism is likely accurate, and it supports an earlier date of writing of the fourth gospel—too early to have been written to counter the Gnostic paradigm’s typical interpretation of dualism.
While the Gnostic heresy would not fully manifest itself until the second century, Docetism, a precursor of the better known Gnosticism, did cause problems for the first century church. A heretic from Egypt, Cerinthus, a contemporary of the Apostle John, was a vocal proponent of this heresy claiming Jesus was not divine but human, born of Joseph and Mary, and did not become the Messiah until His baptism at the age of thirty. He also taught that Jesus had a dual nature, physical and the spiritual, which was the Messiah. At death, the Messianic spirit left the physical, and there was no bodily resurrection. Cerinthus also taught his followers to receive mystic experiences and higher knowledge of God apart from the Apostles and Scripture. To counter these teachings, the fourth gospel notes several times that Jesus is the manifestation of God in the flesh and divine.
This also explains why the gospel includes verses such as John 1:14, 4:5-6, and 19:34-35, telling how the Word became flesh and the various human aspects of Jesus. Early church leaders also opine the reason for the gospel. According to Clement of Alexandria (150-215 CE), the Synoptics more than adequately detail the external or historical aspects of the ministry of Jesus; he suggests the purpose of the fourth gospel is to examine the spiritual aspect of the ministry of Jesus. Secondary motives are to encourage those who believe in Jesus and to explain to believers who are facing rejection by their friends and families that Jesus willingly endured similar treatment.
A new believer can read the gospel of John and gain much insight into the Christian faith, yet not be intimidated. At the same time, this gospel contains spiritual truths that confound theologians. As John 20:30-31 explains, do not view the gospel as a biography of Jesus but as personal, eyewitness testimony with the express goal of providing information to you to make a rational decision about Jesus. This book is not an exercise in theology or doctrine, although it does contain both in great measure. The basic premise of the author can be seen in John 1:11-13 within the introduction to the gospel.
These three verses, if you read the book of John from this perspective, will give great insight into the message and meaning of the fourth gospel. This gospel reveals the person of Jesus and that He is the Messiah. It is an evangelistic gospel meant to bring readers to a saving relationship with Jesus. Gerald Borchert writes, “while the purpose statement is clearly linked to a missionary focus, the format of most of the pericopes in the gospel is styled according to a teaching format.” 
John's gospel contains strong doctrinal and theological teachings that center on the person and role of Jesus. Of all of the gospels, no other presents the divinity of Jesus as strongly. “The viewpoint of this gospel is more elevated than that of the others; its contents bring into view spiritual relationships rather than human ties; and, higher glories are revealed, as touching the peerless Person of the Savior.”  The author utilizes Old Testament doctrines and prophecies to explain how Jesus is the fulfillment. The introductory phrase, “In the beginning…” reminds one of the first words in the Old Testament. In this way, the gospel presents Jesus as the way to restore the relationship between the Creator and the created as it was in the beginning. The use of the word “logos” was the verbal device John chose to introduce this concept.
A literary technique in the gospel is the application of dualism to contrast two things. The use of contracting points or paradigms was a common method at the time of Jesus to communicate ideas and concepts. This technique can be seen in pagan, Gnostic, Essene, and other writings. In the fourth gospel are many dual comparisons including light and dark, love and hate, truth and lie, God and Satan. John also uses the judicious application of irony both in the conversations and in the recounting of events. John also frequently uses symbolism to explain Jesus and His ministry. Also, he describes Jesus as the logos, bread of life, shepherd, and many other things. These are meant not so much to describe Jesus but to create word associations in the mind of the reader. Unfortunately for us, it is difficult to discern the irony and to fully understand the word pictures found in this gospel because of the vast differences between the world of today and of the time of Jesus. The culture, social norms, and political environment are so different it is necessary to have some understanding of the world then to grasp better the ministry and message of Jesus. My first book, The Messiah and the Kingdom of Heaven seeks to provide a solid understanding of the world at the time of Jesus. These two books would also be beneficial to understand better the message of this book.
The fulfillment by Jesus of the symbolism found in the Jewish feasts and institutions is another theme of John. Even though the readership of this gospel includes Gentile Christians, there is sufficient information about the Jewish feasts to enable the non-Jewish reader to understand the significance and relationship between Jesus and the feasts. Also, Matthew, Mark, and Luke provide more material about the Jewish religion. As the light of the world and the living water, Jesus fulfills the Feast of the Tabernacles’ lighting of the torches and the pouring of the water, for example. The signs or miracles were other methods of teaching. Each sign is stronger than the previous, and each reveals more of His divine power and authority. The first sign, turning water into wine, demonstrates His power over quality; the second sign, the healing of the nobleman’s son, reveals His power over distance; the third sign, healing the man crippled for 38 years, shows His power over time; the fourth sign, the feeding of the 5,000, manifests His power over quantity; the fifth sign, walking on the water, reveals His power over natural law; and the sixth sign, the healing of the man born blind, demonstrates His power over misfortune.
The signs culminate in the resurrection of Lazarus to reveal His mastery of death. The “I am” statements reveal still more aspects of Jesus and present Him as the fulfillment of the Jewish hope. His crucifixion during Passover fulfills the symbolism of the slain lamb, and the resurrection fulfills the symbolism of the Jewish Temple and the sacrificial system. One can argue that every sign and teaching of Jesus relates to some aspect of Judaism. John echoes this in 1 John 1:1-3 when he reflects on the reliability of what he had written. While there were certainly other reasons for writing the fourth Gospel, John 20:30-31 should be taken at face value: to encourage its readers to believe in Jesus Christ and to receive salvation and eternal life.