Jesus gives two metaphors to describe disciples--salt and light.
Why salt? What characteristics of salt are displayed in discipleship? Salt preserves, keeps, protects. And, salt improves the taste of something. Was Jesus saying that the church was to do these things?
"You are salt," he said. Then he went on to remind them that if salt didn't perform the functions it was intended for, then is was not of any use and would be thrown out.
Is the church--or substitute "your congregation"--preserving, keeping, and protecting, or, making things better (Note: go back and read verses 1-12 to aid in answering)?