One Way to Look at It: Light vs. Fire

If you ask a Christian what their responsibility is toward the world, many would use the image of light.  Thanks to the popular children’s song, we all know we need to let it shine and not hide it under a bushel or let the devil blow it out.  Jesus used this idea to clarify for his followers their purpose, which is the same for us now.  
Be a light.
What does that mean?  Well, what does light do?  It touches everything nearby, showing the truth of what it is.  Only in darkness can anything hide; light defines and causes serves to illuminate the existing reality.  
The problem is when we attribute tasks to ourselves that have nothing to do with that task.  What does light not do?  It doesn’t cause change.  It doesn’t purify.  It doesn’t harm or benefit whatever it shines on, besides the natural consequence of being shown.  
Fire does that. Fire changes whatever it touches. Fire purifies. It does cause harm or benefit, depending on the substance. Light is the result of fire, not the other way around.  
God presents his power through fire throughout the Bible, as He led the Israelites through the wilderness as a pillar of flame.  The Bible describes God as a consuming fire when the Israelites are about to enter the Promised Land. His power over Baal is shown by the sheer force of the fire that He sent down to Elijah’s soaked altar after Baal failed to show up.  The Psalms use imagery of purification by fire.  Shadrach, Meshac and Abednego are unharmed by an extremely hot fire thanks to a holy presence.  
Being fire is God’s sphere.  Being light is ours, and even in that, we do best when we remember that we reflect Jesus and have nothing to show without Him. 
And yet it is easy to spend our effort and attention on trying to do God’s job.  I get it.  It’s natural that we would want things to be better, more in line with His kingdom. But He gave us our part to play and we would do well to trust Him with the rest.

Instead, speaking the truth in love, we will grow to become in every respect the mature body of Him who is the head, that is, Christ.
— Paul, Ephesians 4:15