There’s a common pattern in domestic abuse cases, with the abuser professing his love and apologizing, then losing control and lashing out again, then again professing his love and sorrow, and so forth. In such cases its pretty clear that the external truth – a swinging fist – does not match the love that supposedly exists. On the other hand, when someone professes love and acts in ways that are tender and caring, the external truth – the kind actions – tend to confirm that the love is real.

That’s what the Bible means by being a "witness": It represents using what we know, the truth in front of our noses, to judge the nature of love. In this case, the witness is John, who represents the external, literal meaning of the Bible, and the "light" is divine truth brought to the world by Jesus. So John was re-establishing the Word – the laws and prophets of the Old Testament – so that Jesus could show the perfect love and truth that fill it internally. With the two in harmony, those who were willing would be able to know that Jesus was the Lord, an expression of the ultimate, creative, eternal love.

(Referenser: The Apocalypse Explained 27 [2])