It is commandment.

Jesus does not say this about you as a kind of flattery or tribute to you.  The Christian is not congratulated for being light of the world but is consumed, as a candle is by the flame, so the world will praise not you but your Father in heaven. 

Your good deeds are really His.  The brightness and light which shine as your good work are really His.  He's the one who set you upon His holy hill with this charge - that you shine forth now in every motive, action, word, and thought as pure light. 

There must be no hint of shade, of sin or selfishness in you.  Nothing of you must eclipse the Father or obscure His Name.  No cover must ever hide Him from others who need so much the light of Christ.

Darkness can never share with light.  Everyone knows a lamp is not intended to be put under a bowl.  You are the light of the world.  How can God say this of us, calling us the very same as Christ Himself who is The Light of the World?

Our Lord immediately makes clear the answer: Jesus!

He did not come to quench the Scriptures (the Law and the Prophets) but to fulfill them.  His every motive, action, thought, and word revealed divine light.  It is because God has given us His righteousness and buried the darkness of our sin in His own body that we may now shine "this little Gospel light of mine."

Recently, a guy came to our front door representing an energy company and offering to lock in rates for the next five years.  Since deregulation, there is now competition for energy customers.  He wanted me to find a recent Consumers bill to see if I qualified. 

He asked me, "Don't you want to save on utilities?"  I didn't understand half of what he told me, but I did think of my dad who would have said, "The easiest way to save on utility costs is just to turn off the lights." 

We could save on the cost of being a Christian too, I suppose, if we just turned off the light, buried our witness under a bushel basket, and allocated only a few good works to the most essential occasions.

But what child of God would do such a thing when the cost has already been paid!  Jesus fulfilled all the requirements, did all the good work, carried all the cost, and defeated all the darkness.  So now, our heavenly Father says you don't have to conserve your good deeds.

Spend them; use them up; expend them; splurge them; even waste them on others.

You are the light of the world.  Shine!  And people will praise your Father in heaven.