Christian's Ultimate Life Purpose is to Spread the Gospel
You are the salt of the earth.  But if the salt loses its saltiness, how can it be made salty again?  It is no longer good for anything, except to be thrown out and trampled by men. 

You are the light of the world.  A city on a hill cannot be hidden.  Neither do people light a samp 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 maen, that they may see your good deeds and praise your Father in heaven.
Matthew 5:13-16