8:12 I am the light. This is the second of the great “I am” statements of Christ in the gospel of John (see John 6:35), comparing Himself in effect to the sun. Physically speaking, the sun is the life and light of the world (note Genesis 1:14-16, also John 1:4,9; Revelation 21:23-25; 22:5). Christ did indeed create the sun and even now sustains its life-giving radiations (Hebrews 1:3).
8:12 walk in darkness. This is the first New Testament reference to the spiritual “walk” of the believer, and it is important to note that the “walk” is to “follow” the steps of Jesus (compare I Peter 2:21), and to be “in Him” (Colossians 2:6). In the Old Testament, the first such reference is when Enoch, then Noah, “walked with God” (Genesis 5:24; 6:9). The believer is further commanded to “walk in the light” (I John 1:7), “in wisdom” (Colossians 4:5), “in love” (Ephesians 5:2), “in newness of life” (Romans 6:4), “in the Spirit” (Galatians 5:16), “in the truth” (III John 4), and “in good works” (Ephesians 2:10). On the other hand, he is not to walk “according to the course of this world” or “as other Gentiles walk, in the vanity of their mind” (Ephesians 2:2; 4:17).
8:12 light of life. Note the profound prophetic significance of this claim of Christ. It is as strong an assertion of omnipotent deity as one could imagine, but it does not sound conceited or insane, as it would have if it had come from any other man. Rather, it has proved prophetically true for two thousand years. He has been the light of the world—the inspiration for the world’s greatest music and art, its most dedicated hospitals and missions, its greatest and most influential nations and governments. Further, millions of individuals who have followed Him have testified that He was indeed the light of their lives as they walked, not in darkness, but in the light of life.