Here in the introduction to his gospel, John is introducing another person by the same name. This "John" is the man we know as John the Baptist. The beloved disciple wants us to know that this man was sent from God, and that his purpose was exactly the same as John's own purpose for writing -- so that he could witness concerning the light, and people could receive the salvation of the Lord.
John the Baptist wasn't the only person to bear witness about the light. Many others did; the prophets of old proclaimed that Jesus would come. They foretold His suffering and the glories that would follow.
Therefore the Lord himself will give you a sign: The virgin will conceive and give birth to a son, and will call him Immanuel. (Isaiah 7:14)
For to us a child is born, to us a son is given, and the government will be on his shoulders. And he will be called Wonderful Counselor, Mighty God, Everlasting Father, Prince of Peace. Of the greatness of his government and peace there will be no end. He will reign on David's throne and over his kingdom, establishing and upholding it with justice and righteousness from that time on and forever. The zeal of the LORD Almighty will accomplish this. (Isaiah 9:6-7)
Surely he took up our pain and bore our suffering, yet we considered him punished by God, stricken by him, and afflicted. But he was pierced for our transgressions, he was crushed for our iniquities; the punishment that brought us peace was on him, and by his wounds we are healed. We all, like sheep, have gone astray, each of us has turned to our own way; and the LORD has laid on him the iniquity of us all. (Isaiah 53:4-6)
But you, Bethlehem Ephrathah, though you are small among the clans of Judah, out of you will come for me one who will be ruler over Israel, whose origins are from of old, from ancient times. (Micah 5:2)Even John the Baptist himself figured in the prophetic words of Isaiah, who told of his coming to witness and prepare the way for Jesus:
A voice of one calling: "In the wilderness prepare the way for the LORD; make straight in the desert a highway for our God. (Isaiah 40:3)The other three gospels identified John the Baptist as the special witness to the coming of Christ, in their first few chapters. And John the Apostle noted that John the Baptist called Jesus the "Lamb of God" and the "Son of God."
All of these witnesses were prior to the ministry of Jesus here on earth. Tomorrow we will study more about "bearing witness" to the Light.