The Light Of The World

Ye are the light of the world. This also bespeaks them useful, as the former (Sole et sale nihil utilius--Nothing more useful than the sun and salt), but more glorious. All Christians are light in the Lord (Ephesians 5:8), and must shine as lights (Philippians 2:15), but ministers in a special manner. Christ call himself the Light of the world (John 8:12), and they are workers together with him, and have some of his honour put upon them. Truly the light is sweet, it is welcome the light of the first day of the world was so, when it shone out of darkness so is the morning light of every day so is the gospel, and those that spread it, to all sensible people. The world sat in darkness, Christ raised up his disciples to shine in it and, that they may do so, from him they borrow and derive their light.

This similitude is here explained in two things:
As the lights of the world, they are illustrious and conspicuous, and have many eyes upon them. A city that is set on a hill cannot be hid. The disciples of Christ, especially those who are forward and zealous in his service, become remarkable, and are taken notice of as beacons. They are for signs (Isaiah 7:18), men wondered at (Zechariah 3:8) all their neighbours have any eye upon them. Some admire them, commend them, rejoice in them, and study to imitate them others envy them, hate them, censure them, and study to blast them. They are concerned therefore to walk circumspectly, because of their observers they are as spectacles to the world, and must take heed of every thing that looks ill, because they are so much looked at. The disciples of Christ were obscure men before he called them, but the character he put upon them dignified them, and as preachers of the gospel they made a figure and though they were reproached for it by some, they were respected for it by others, advanced to thrones, and made judges (Luke 22:30) for Christ will honour those that honour him. - Matthew Henry