"You are the salt of the earth; but if the salt loses its flavor, how shall it be seasoned? It is then good for nothing but to be thrown out and trampled underfoot by men. You are the light of the world. A city that is set on a hill cannot be hidden. Nor do they light a lamp and put it under a basket, but on a lampstand, and it gives light to all who are in the house. Let your light so shine before men, that they may see your good works and glorify your Father in heaven."
Salt sticks out. If you add salt to something you can usually taste the flavor of it. We are called to be salt by sticking out. The more we conform to the world, the more we lose our flavor and the more pointless we are as christians.
We are the light of the world because we carry the Light inside of us.