According to the Gospel of John, the first miracle that Jesus performed, inaugurating his three-year ministry, was turning water into wine at a wedding in Cana. John calls it a "sign." A sign is a hint, an evidence of a somewhat hidden truth. What was that truth? John reveals it near the end of his Gospel: "Now Jesus did many other signs in the presence of the disciples, which are not written in this book; but these are written so that you may believe that Jesus is the Christ, the Son of God, and that by believing you may have life in his name" (20:30-31).
This inaugural sign was only witnessed by a few. At the end of John's narration, he says, "And his disciples believed in him." I find it interesting that when Jesus had the opportunity to explode on the scene and make his presence known, he limited himself and only revealed himself to a few. And that few were his disciples.
One of the great blessings of faith that God gives to his followers is something we like to call "assurance." Primarily through the reading of God's word and the conviction of the Holy Spirit, we who believe have this assurance that Christ has saved us by means of his death on the cross. The disciples, all of whom had only recently begun following Christ, we're given this sign to confirm for them that Jesus was the Christ, the Messiah, the one who God had anointed as His Servant (cf. Isaiah 53).
In this passage, although others may have noticed and believed that Jesus was a man of God, John only records that the disciples believed in him. God rewards the faith of his children with assurance, but there is no assurance those who have not taken that initial step of faith, and so they remain in their unbelief, waiting for a sign that God will only give to his own. We too must walk in faith and trust that God, who rewards faith, will give us the gift of assurance.