The Apostle John is usually credited with the authorship of the fourth Gospel. First of all, the author had to have been an eyewitness of the ministry of Jesus (1:14; 19:35; 21:24). He would have also had a decent familiarity with Palestine before the destruction of the temple in AD 70, and would have been familiar with the Jewish way of life. John the Apostle does fit the description, but it is not exclusive to him. Early traditions help to identify the author as John. Irenaeus, a disciple of John’s disciple Polycarp, is of the earliest extant sources to associate John with the fourth Gospel.  Like the other Gospels, the title “According to John” (KATA IWANNHN) is found in the earliest manuscripts.
John and his brother James, accompanied by their father Zebedee, were preparing their nets in a boat when Jesus called them. Both James and John left the boat and their father to follow Jesus (Matt 4:18-22). Jesus often took Peter, James, and John aside defining them as an inner circle of disciples (13:23-24; 20:2-10; 21:2, 7, 20ff.). Not only is John counted among this select group, but he also refers to himself as the disciple whom Jesus loved (13:23; 20:2; 21:7, 20). After the resurrection of Jesus, John continued to play an instrumental role in the early church. Paul referred to Peter, James, and John as the pillars of the church in Jerusalem (Gal 2:9). John is found with Peter going to the temple when Peter healed the crippled man (Acts 3:1-11). As a result both men went before the Sanhedrin (Acts 4:1-23). Peter and John later traveled up to Samaria to confirm the work that was going on there (Acts 8:14-24).
Date and Location of Composition
John’s Gospel is generally considered to be the last of the four canonical Gospels to be written. On the other hand, as Michaels points out, “no limit could be fixed on how early it could have been written” because John does not rely on the Synoptic Gospels.  In the past, some have dated the Gospel in the second century, but that view has decreased in popularity after the discovery of two important manuscripts of John’s Gospel that are dated in the early part of the second century (P46 and Egerton Pap. 2). The majority of scholars date the Gospel in the period AD 90-100, though some have dated it much earlier. 
The location of John’s Gospel has recently been disputed. The two possibilities that have gained the most acceptance are Syria and Asia Minor. Syria is mentioned because of the Gospel’s connection with the Odes of Solomon  and Ignatius of Antioch (ca. 110), who had early association with John’s Gospel. On the other hand, early church tradition suggests that that John composed his Gospel in Ephesus (Asia Minor). An example of this is the testimony of Irenaeus: “Then John, the disciple of the Lord, who had even rested on his breast, himself also gave forth the Gospel, while he was living at Ephesus in Asia.” 
Purpose and Audience
John specifically states his purpose in 20:31, “But these are written that you may believe that Jesus is the Christ, the Son of God, and that by believing you may have life in his name.” Therefore the purpose of John’s Gospel is to “confirm and secure Christians in the faith.”  John also uses 3:16 and 8:24 to support this. Eusebius argued that John wrote in order to complement the Synoptics where they were lacking,  while the Muratorian Canon suggested that his fellow disciples in Asia Minor urged him to write an account. 
The specific recipients are not clearly spelled out in the Gospel. Conjecture, along with the Muratorian Canon, would suggest that it was written for the disciples in Asia Minor, but there is no certainty in this.
Themes and Theology
John presents man as either belonging to one of two things: the darkness or the light. There is no in between. The darkness is associated with death, while the light is associated with life. This theme is developed throughout the Gospel. In 1:4-9, John portrays Jesus as being the light of men and demonstrates that the darkness does not understand the light. John the Baptist came to bear witness of the light in order that men would believe through him. In the third chapter (19-21) Jesus states that the light has come into the world, but men have loved the darkness instead of the light because their works were evil. Evildoers hate the light and are afraid to go into the light lest their works be exposed. On the other hand, the ones who practice the truth come into the light so that it can be seen that their works were done through God. In 5:35 there is reference to John the Baptist as being a lamp that gives forth light. Jesus is also referred to as the Light of the World (8:12; 9:5) whereas the devil is called the “prince of this world” (12:31; 14:30; 16:11). In 12:35-36 Jesus tells the crowd that it is necessary to walk in the light because the person that walks in the darkness does not know where he is going. He also tells the crowd to believe in the Light in order to become sons of the light. Lastly in 12:46, he states that he has come as light into the world so that those who believe in him will not remain in darkness.
The Gospel of John also develops a Christology that is unique from the other Gospels. One of the overriding themes throughout the New Testament is that Jesus is the Messiah. In presenting this, John’s Gospel also makes it clear that Jesus is God. In the opening verse (1:1), John plainly declares that in the beginning Jesus (the Logos) was with God and was God. Throughout the Gospel many references are made to Jesus’ deity. Most notable of these are 8:57-58 where Jesus declares, “Before Abraham was born, I am (egw eimi)!” It is often thought here that Jesus is making an allusion to Exod 3:14 (LXX) where God refers to himself as “I am (egw eimi) the one being/existing (o wn).” Another indication of Jesus’ deity can be found in 10:30-33. In this instance Jesus said, “I and my Father are one.” After this, the Jews picked up stones that they might stone him on grounds of blasphemy because he claimed to be God. At the end of the Gospel (20:28) Thomas makes a climacteric confession of faith by declaring that Jesus as his God.
The prologue to the Gospel begins by saying, “In the beginning was the word (logos), the word (logos) was with God, and the word (logos) was God.” As it is translated here, the word logos is usually translated as “word.” Logos has a deep philosophical tradition that antedates John’s Gospel by half of a millennium. Heraclitus (560 BC) first used the word in reference to a fixed principle in a world of change; it was his expression of God.  The Stoics later adapted a similar principle that the Logos was the power that controlled and ordered the world. The Logos would have helped John to present Christianity to Greeks who were familiar with the idea.  However, this idea of the word in relation to God was not an exclusively Greek principle. Philo (a first century Jew from Alexandria), being influenced by the Greeks, related the idea of the Logos to Yahweh, the God of Israel.  Additionally, the Hebrew Scriptures almost certainly inspired John to use the Logos. When the ‘word of God’ is used throughout the Old Testament, it often refers to God being in action, particularly in regards to “creation, revelation and deliverance.”  With the use of Logos John would have been able to reveal Christ’s deity to both Jews and Greeks alike. The majority of the occurrences of logos in John take place in a “syntactical sequence with Jesus or God.”  John uses logos in direct reference to the person of Christ (1:1, 14), his message (4:50; 5:24; 15:3, et al.), and within his message (4:47; 5:38; 17:17, et al.).
Literary Style, Structure, and Other Issues
The Gospel of John varies from the Synoptic Gospels in more ways than one. J. Ramsey Michaels categorizes them into two types of variation: (1) the style and content of Jesus’ teaching, and (2) the chronology and structure of Jesus’ ministry.  Another characteristic that set John apart is his writing style. Concerning this, Clement of Alexandria stated that John was concerned with details and wrote a “spiritual gospel.” 
Chapter 21 is commonly called the epilogue. On the surface this section (21:1-25) seems a bit out of place because the last couple of verses in the twentieth chapter seem to bring the Gospel to a close. This has led some to believe that the epilogue was a later addition by John or one of his disciples. There is, however, no reason to think that John did not write it. There are 28 words in the last chapter that are not found elsewhere in the Gospel, but most of these are caused by the subject-matter in the first 14 verses.  It is also composed in classic Johannine style and there are no complete manuscripts of John that do not contain the epilogue. More importantly, as one commentator put it, it “completes” the Gospel by “illustrating the result of belief.”