In the Old Testament God promised that He would send a special servant that would be a light and bring light into a dark world.
Jesus is the source of absolute truth and moral purity.
As Christians, we must continually combat the moral and spiritual decay around us by being a light in a dark world.
By being a light in this world, we as Christians show the beauty of Christ and the gospel by our words and actions.
Unfortunately, throughout the centuries this portion of the Sermon on the Mount that deals with salt and light has been abused, misinterpreted, and misapplied.
We are called to be a light in this world and can practically do so by displaying godly character, accomplishing good works, and proclaiming the gospel of Christ.