The first eighteen verses of John’s Gospel function as a prologue to the entire work. Far more than a mere literary convention, these words serve as a lens by which the rest of the work is properly understood. In these verses, John introduces Jesus as the central figure around which the drama and theological themes will revolve. From the beginning, the apostle leaves no doubt as to the identity and cosmic significance of Jesus Christ. “Although it is technically a prologue, it is also a narrative, the narrative of the Word, Jesus Christ, the subject of the biography. This narrative introduction tells the reader who Jesus is and what Jesus has done; the rest of the Gospel will explain how Jesus acts on behalf of God in the human story” (Klink, p. 81).
In the prologue, John includes key words which serve to highlight the book’s central themes. Words such as light, life, witness, sign, true, world, and glory are found in the prologue. “But supremely, the Prologue summarizes how the ‘Word’ which was with God in the very beginning came into the sphere of time, history, tangibility – in other words, how the Son of God was sent into the world to become the Jesus of history, so that the glory and grace of God might be uniquely and perfectly disclosed. The rest of the book is nothing other than an expansion of this theme” (Carson p. 111).
John depicts the world as a place plunged into spiritual darkness. Jesus is therefore the light which shines in that very darkness. This is so because Jesus is the ultimate self-revelation of God in human flesh (the Word). And because God came into the world in the Person of Jesus, the light of salvation now shines to the those trapped in sin’s darkness. And no matter how great the darkness, Jesus the Light is able to pierce through. Through Jesus, sinners are called to be children of God on the basis of faith alone. In this way, John’s prologue functions not only to introduce his Gospel account but is itself a proclamation of the Gospel. God sent his Son into the world that he loves, in order to save sinners.
 Edward Klink, John, ECNT (Grand Rapids: Zondervan, 2016) p. 81