Today's Gospel reading is the one from Matthew about being the salt of the earth. Some years ago I was at a workshop given by a speaker named John Pilch. He's studied the cultural world of the Bible and gave a great explanation of this passage. He said that in the ancient Mideast, people used animal manure as fuel. They would make it into little "cakes" and they would put salt in it as a catalyst to start the fire. So when Jesus said, "You are the salt of the earth," he knew salt was mainly used for that purpose--the catalyst to start the fire. Then the next metaphor, "you are the light of the world," makes even more sense. First the fire, then the light. So Jesus is really telling us that we have to set the world on fire for God.
A story about St. Dominic says that when his mother was expecting him, she had a dream. In it, she saw herself giving birth to a dog with a torch in its mouth that would set the world on fire. I don't know if the story is true or not, but it's an interesting twist on the theme of faith as a fire.