In the time of Jesus salt was connected in people's minds with some special qualities.
Salt was connected with purity. No doubt its glistening whiteness made the connection easy. The Romans said that salt was the purest of all things, because it came from the purest of all things, the sun and the sea. Salt was indeed the most primitive of all offerings to the gods, and to the end of the day the Jewish sacrifices were offered with salt. So then, if the Christian is to be the salt of the earth he must be an example of purity.