10 To this point the author has used past tenses (imperfects, aorists); now he switches to a present. The light continually shines (thus the translation, “shines on”). Even as the author writes, it is shining. The present here most likely has gnomic force (though it is possible to take it as a historical present); it expresses the timeless truth that the light of the world (cf. 8:12, 9:5, 12:46) never ceases to shine. The light shines on. The question of whether John has in mind here the preincarnate Christ or the incarnate Christ is probably too specific. The incarnation is not really introduced until v. 9, but here the point is more general: It is of the very nature of light, that it shines.
11 The author now introduces what will become a major theme of John’s Gospel: the opposition of light and darkness. The antithesis is a natural one, widespread in antiquity. Gen 1 gives considerable emphasis to it in the account of the creation, and so do the writings of Qumran. It is the major theme of one of the most important extra-biblical documents found at Qumran, the so-called War Scroll, properly titled The War of the Sons of Light with the Sons of Darkness. Connections between John and Qumran are still an area of scholarly debate and a consensus has not yet emerged. See T. A. Hoffman, “1 John and the Qumran Scrolls,” 8 (1978): 117–25.
12 Grk “and,” but the context clearly indicates a contrast, so this has been translated as an adversative use of καί (kai).
13 Or “comprehended it,” or “overcome it.” The verb κατέλαβεν (katelaben) is not easy to translate. “To seize” or “to grasp” is possible, but this also permits “to grasp with the mind” in the sense of “to comprehend” (esp. in the middle voice). This is probably another Johannine double meaning—one does not usually think of darkness as trying to “understand” light. For it to mean this, “darkness” must be understood as meaning “certain people,” or perhaps “humanity” at large, darkened in understanding. But in John’s usage, darkness is not normally used of people or a group of people. Rather it usually signifies the evil environment or ‘sphere’ in which people find themselves: “They loved darkness rather than light” (John 3:19). Those who follow Jesus do not walk in darkness (8:12). They are to walk while they have light, lest the darkness “overtake/overcome” them (12:35, same verb as here). For John, with his set of symbols and imagery, darkness is not something which seeks to “understand (comprehend)” the light, but represents the forces of evil which seek to “overcome (conquer)” it. The English verb “to master” may be used in both sorts of contexts, as “he mastered his lesson” and “he mastered his opponent.”