Yet because you are not of the world, but I chose you out of the world, therefore the world hates you. In Jesus' time, salt was a valuable asset. To be a Christian means to preserve society. We are in the world and are meant to be among our neighbors sprinkling salt. If our light is hidden under a basket or shade, it isn't doing its job.
Light reveals darkness.
The world doesn't know that it is living in darkness unless it can see itself in contrast to the light. If you are a Christian, then your light, as you live your life, will shine on those dark areas that need illumination. There must have been a rumor circulating throughout Galilee that Jesus posed a threat to the Law and the Prophets. Jesus sets the record straight immediately by calming their fears and assuring the crowd that his aim is not to destroy all that has come before Him. Jesus said that those who break and teach others to break the Law will be called least in the kingdom of heaven.