A city that is set on an hill cannot be hid. There are two clear principles stated for Christians in this passage. The first is the Salt Principle which, simply stated, is salt preserves from corruption. When the New Testament was written, salt was the primary preservative for meat. God s people are left in this world to witness against iniquity and to set an example for righteousness.
Christians are to be the moral conscience of society.
The second principle is the Light Principle. In His physical absence, His disciples are to witness for Him in the darkness of this world. Simply put, Christians should see to it that the light of revelation is made available to everyone. However, the glorious gospel of grace is not self-evident. Left to themselves, men always devise a system of salvation by works.