The gospels of Matthew and Luke both begin with infancy narratives, while Mark begins with Jesus as an adult just prior to his baptism by John the Baptist. However, the gospel of John begins very differently. John begins with a prologue, which introduces the central theme of his gospel. This prologue, or theological introduction, describes Jesus as the pre-existent, eternal Word of God that has now become incarnate (the Word became flesh).
When John describes Jesus as the Word of God, he is specifically referencing the Greek philosophical concept of the Logos. The Logos, in Greek thought, represented divine wisdom or reason, which brought order and purpose to the universe. John makes the theological claim that the Logos in fully divine (the Word was God) and that the Logos was now incarnate in the person of Jesus. It is often difficult for modern readers to grasp the theological depth of these claims, since we are so far removed from the Greek philosophical world from which these concepts were drawn. Nevertheless, the prologue of John's gospel clearly presents Jesus as the fully divine Son of God who has come into the world to live and die as one of us. The doctrine that Jesus was both fully human and fully divine is an essential part of the Christian faith.
Like the other gospel writers, John connects the life of Jesus to the history of God's people found in the Old Testament. The opening sentence of John's gospel ("In the beginning...") mirrors the opening lines of the book of Genesis. Jesus is described as the fulfillment of the law of Moses (1:17). However, the most striking allusion to the Old Testament is often lost in most English translations. In verse 14, John famously declares, "The Word became flesh and dwelt among us." The Greek word that is translated "dwelt" literally means "to camp in a tent." This is a reference to the Old Testament tabernacle, the tent that was used as a house of worship during the Israelite's journey through the wilderness of Sinai. The tabernacle/tent was ultimately replaced by the permanent Temple in Jerusalem under the leadership of King Solomon. The theological point that John is attempting to make is that Jesus is the very presence of God dwelling on earth. Just as the presence of God was present in the tabernacle and, subsequently, the Temple, so now the presence of God has become incarnate in the person of Jesus.