He himself was not the light; he came only as a witness to the light.
The writer again uses repetition to emphasize his point: John the Baptist was not the Messiah, but he was an emissary from God with a very important mission, to be a witness as to the real Messiah of God.
Even though he was not the light, Jesus later says that “John was a lamp that burned and gave light” (John 5:35). The purpose of a lamp is to let the light be seen, and that is exactly what John did. What about you?
John 5:35 John was a lamp that burned and gave light, and you chose for a time to enjoy his light.