John 8:12

Again therefore Jesus spake unto them, saying, I am the light of the world: he that followeth me shall not walk in darkness, but shall have the light of life.

JESUS; THE LIGHT OF THE WORLD

I am the light of the world ... is the second of the seven great "I am's" of John. See: John 6:35; 8:12; 10:9; 10:11; 11:25; 14:6; and 15:5.

Several suggestions of what might have prompted such a metaphor by Jesus are: (1) the great lamps kindled in the temple court during the feast of tabernacles, (2) the glorious sun rising at that very moment over the mount of Olives, and (3) the pillar of fire that lighted the way for Israel in the wilderness; but it seems more reasonable to suppose that if Jesus needed any suggestion of such a metaphor he would have rather found it in the "light" passages of the Old Testament. Note:

I will also give thee for a light to the Gentiles, that thou mayest be my salvation to the end of the earth (Isaiah 49:6). I the Lord have called thee in righteousness, and will hold thine hand, and will keep thee, and will give thee for a covenant of the people, for a light of the Gentiles (Isaiah 42:6). But unto you that fear my name shall the Sun of righteousness arise with healing in his wings (Malachi 4:2).

As the sun is the source of all light, power, and energy on earth, Jesus the Sun of righteousness is the source of all spiritual light, power, and energy. Light is the only thing that can come into contact with filth and remain uncontaminated. Christians are the "light of the world" (Matthew 5:14), but theirs is a reflected light. Men of righteous intention seek the light (John 3:19ff). Light either kills or develops vegetation, depending on whether or not it is rooted in soil; and the gospel has that same dual function (2 Corinthians 2:15ff). Light is its own witness. See next verse.