"He came as a witness, to testify about the Light, so that all might believe through Him."
John was sent out by God as one having already seen the Light to those still in darkness. Imagine being sent to a people who lived underground in complete darkness with the message, "Come to the light!" They would not understand. All they've ever seen is darkness; they don't know what light is. Trying to expalin it to them would be like explaining color to a blind person. This is what we are called to do. We, like John, have seen the Light, but we all have friends and relatives, neighbors and co-workers who are still living in darkness. It is not our job to convince them that there is light. We are simply called to testify about what we have seen. We are to bear witness about the Light that we have seen. We cannot convince the cave-dwellers that there is light while they sit in darkness. Our job is only to arouse their curiousity, make them say, "What if he's right?" because the next step is always, "I need to see this for myself." Once they leave they darkness and enter the Light, our job is done because the Light speaks for itself.