This beginning of his signs Jesus did in Cana of Galilee, and revealed his glory; and his disciples believed in him.

 John does not relate the story of any wonder that Jesus did in Jerusalem at the Passover season; but Jesus did do wonders there; and there were many who, when they saw his powers, believed in him. The question John is answering here is--if there were many who believed in Jerusalem right at the beginning, why did Jesus not there and then set up his standard and openly declare himself? 