John's Gospel was written by "the disciple whom Jesus loved" (Jn 21:7,20,24). He was an eye-witness to Jesus' life, death and resurrection (19:26; Acts 1:21,22). According to chapter 20 verse 30, there are many things about Jesus and many events which he did not have the space to include in this gospel. He is selective in his material. His purpose in writing this gospel is stated in John 20:31: "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."
John's Gospel is deductive. John announces in the prologue that Jesus is the eternal Word--God who was in the beginning, the Word through whom all things were made. He is God who became flesh and made his dwelling among us. There are 7 miracles or signs: Ch 2,4,5,6 (2),9,11). There are 7 "I Am's" that point to his divine Messiahship; 6:35; 8:12; 10:7,11; 11:25; 14:6;15:5.
Belief/unbelief is a major theme. In chapters 1-4 he shows what belief is; in chapters 5-12, he shows why people don't believe. Other themes are: darkness/light, life/death, witness or testimony, love, glory.
John 1:10-12 says: “He was in the world, and though the world was made through him, the world did not recognize him. He came to that which was his own and his own did not receive him. Yet to all who received him he gave the right to become children of God.”
Chapters 1-4 are about his witness to the world; (“...the sin of the world (1); God so loved the world (3) The Savior of the world. (4) Chapters 5-12 are about his witness to the Jews-his own people–and their rejection. Chapters 13-17 are about Jesus’ witness to those who received him and believed on his name–his disciples. Chapters 18-21 concern Jesus’ death and resurrection.
So, John's Gospel may be divided as follows:
1:1-18 - Prologue, The Eternal Word
1:19-4:54 - The Lamb of God who takes away the sin of the world