But that is not light’s only quality. It also measures. Did you ever watch a man pick up a stick and sight along it? What is he doing? He is trying to see whether it is crooked. What reveals that? A beam of light. Light is straight and anything that does not correspond to it is crooked. Surveyors use light to measure distances and angles, to see whether they are up or down, high or low, right or left. They have an instrument with a small telescope on it which uses light as a measurement. Also, in the vast, illimitable reaches of space today the only adequate measuring stick is light-years, the distance measured by the speed of light.

That is what light does, and that is what God does. God is a measuring stick, a point of reference. You can use God to measure everything else. In economic life, political life, social life, scientific life, psychological life, whatever it may be, we are confronted with mysteries and puzzles wherever we turn. As men seek to ferret out the solutions to these puzzles they come up with many proposed solutions. Some are contradictory, some are supplementary to each other, some are absurd, some are stupid, some are very appealing and practical. Every one of us, facing this welter of advice, is constantly asking, how do I know which one is right? How do I know who has the real answers? Where do I get a measuring stick that can be applied to these voices I hear? That is where God comes in. 

(Ray C. Stedman, Expository Studies in 1 John: Life by the Son, Waco, Texas: Word Books, 1980; From Chapter 2, God is Light, 27)