D. Martyn Lloyd-Jones - The Studies in the Sermon on the Mount (Ch. 15)

"You are the light of the world." (Matthew 5:14)

The worst things always happen under cover of darkness; even the natural man, degenerate and in a state of sin, would be ashamed of such things in the glare of light. Why? Because light exposes; 'Whatsoever doth make manifest is light.'

Now the Christian 'is the light of the world' in that way. It is quite inevitable, he cannot help it. Just by being Christian he shows a different type of life, and that immediately reveals the true character and nature of the other way of living. In the world, therefore, is like a light being put on, and immediately people begin to think, and wonder, and feel ashamed. The more saintly the person, of course, the more obviously will this take place. He need not say a word; just by being what he is he makes people feel ashamed of what they are doing, and in that way he is truly functioning as light.

reference: Studies in the Sermon on the Mount

by David Martyn Lloyd Jones - 1971