12. I am the light of the world—As the former references to water (Jn 4:13, 14; 7:37–39) and to bread (Jn 6:35) were occasioned by outward occurrences, so this one to light. In “the treasury” where it was spoken (see on Jn 8:20) stood two colossal golden lamp-stands, on which hung a multitude of lamps, lighted after the evening sacrifice (probably every evening during the feast of tabernacles), diffusing their brilliancy, it is said, over all the city. Around these the people danced with great rejoicing. Now, as amidst the festivities of the water from Siloam Jesus cried, saying, “If any man thirst, let him come unto me and drink,” so now amidst the blaze and the joyousness of this illumination, He proclaims, “I AM THE LIGHT OF THE WORLD”—plainly in the most absolute sense. For though He gives His disciples the same title, they are only “light in the Lord” (Eph 5:8); and though He calls the Baptist “the burning and shining light” (or “lamp” of his day, Jn 5:35), yet “he was not that Light, but was sent to bear witness of that Light: that was THE TRUE LIGHT which, coming into the world, lighteth every man” (Jn 1:8, 9). Under this magnificent title Messiah was promised of old (Is 42:6; Mal 4:2, &c.).
he that followeth me—as one does a light going before him, and as the Israelites did the pillar of bright cloud in the wilderness.
but shall have the light of life—the light, as of a new world, a newly awakened spiritual and eternal life.
Robert Jamieson, A. R. Fausset, and David Brown, Commentary Critical and Explanatory on the Whole Bible, vol. 2 (Oak Harbor, WA: Logos Research Systems, Inc., 1997), 143.