You are the light of the world.
A city on a hill cannot be hidden.
Neither do people light a lamp and put it under a bowl.
Instead they put it on its stand, and it gives light to everyone in the house.
In the same way, let your light shine before men, that they may see your good deeds and praise your Father in heaven.
Notice that it says, "You are..." It does not say that this is what we should be. We are lights. We shine in a dark world. Consider the words of John:
In the beginning was the Word, and the Word was with God, and the Word was God. He was with God in the beginning.
Through him all things were made; without him nothing was made that has been made. In him was life, and that life was the light of men. The light shines in the darkness, but the darkness has not understood it.
There came a man who was sent from God; his name was John. He came as a witness to testify concerning that light, so that through him all men might believe. He himself was not the light; he came only as a witness to the light. The true light that gives light to every man was coming into the world.
John 1:1-9 (NIV)
So Jesus, who came into the world as a light, tells us that when we are his, we will shine that light. This is a light that cannot be hidden. And it would be ridiculous to even think that. "A city set on a hill cannot be hidden!" Cannot!
Also, the person who lights the light would not hide the light. And neither would Christ, who is the lighter of all lights, hide the lights he has set out in this world. He has put us here so that people may see our good works and glorify God!
Consider also these later words of John:
This is the message we have heard from him and declare to you:
God is light; in him there is no darkness at all.
If we claim to have fellowship with him yet walk in the darkness, we lie and do not live by the truth.
But if we walk in the light, as he is in the light, we have fellowship with one another, and the blood of Jesus, his Son, purifies us from all sin.
I John 1:5-7 (NIV)
Claiming to be a light is not enough. People can say anything. They can even say with their mouths that they are in the light. But if their lifestyle is in the darkness, then are they truly in the light? No.