Christ says we are to "let" our lights shine. In other words, when you get right with God and are filled with the Holy Spirit, He will simply shine through you. God is the light that shines within and through His church as the light of the world. The same word the Bible uses for light in Matthew 5 to describe Christ’s Church also describes Christ Himself in John 8:12.
You have been saved out of the world and then sent back into the world to witness to the world, and that's the only business in the world you have in the world, till you're taken out of the world! How can you? By following these four rules.