What is the nature and function of the prologue of the Gospel of John? This article contends that John 1:1-18 provides the exegetical key to the right understanding of the entire book. It contains a summary of the main theological positions of John. The prologue is a "microcosm of the gospel" that anticipates John's presentation of God's purposes of salvation through the incarnation of the Son. Major themes of the Gospel are already present in the prologue, such as the logos, the Word as eternal life, light in conflict with darkness, faith and belief, glory and glorification, grace and truth, and unbelief of Israel.