Jesus uses a similar parable a couple other times. The salt in the area came from the dead sea and much of it was mixed with other compounds other than salt and was actually a liability, what can you do with it...not preserve anything or use it as fertilizer. How could Jesus apply this to today? He might be talking about is watered down Christianity. No essential substance, it's worthless. The modern church has more Bibles than any Church in history, but a essentially illiterate....we need to protect our saltyness.