What does John 8:12 mean?

Seven times in the gospel of John, Jesus makes a powerful statement beginning with the phrase "I am." These comments echo the words of God to Moses in Exodus 3:14. When Moses asks who he should say has sent him to Israel, God tells Moses to tell the people "I AM WHO I AM…Say this to the people of Israel: 'I AM has sent me to you.'" This phrase implies the simplest expression of God's nature: He just is, He must be. When Jesus uses this phrasing, He is deliberately invoking that same essence.

This incident occurs during the Feast of Booths in Jerusalem. Jesus has already used festival rituals as analogies for His role as Messiah (John 7:37–38). As part of this major festival, lamps would be lit using wicks made from priestly garments. In addition, light was a powerful metaphor in Hebrew thinking (Psalm 84:11; Malachi 4:2). "Light," for the Jewish person, was the ultimate ideal and a representation of salvation, knowledge, and goodness. For Jesus to claim to be the "light of the world" was no small thing. In fact, it is a claim to equality with God. Even further, the Greek of this passage indicates Jesus claims to be "the" light, not merely "a" light. In the text's original Greek, Christ says "Egō eimi to phos tou kosmou," which explicitly claims He is the single, solitary source of "light."

Those who follow Christ, in truth, may stumble into darkness, but they will never perpetually "walk" in it (John 12:46; 1 John 1:5—2:2; Psalm 36:9). Light illuminates and uncovers; the only way to stay in darkness is to avoid the light entirely.

This is the second of John's seven "I am" statements. The first came when Jesus was preaching to the crowd in Capernaum, the morning after feeding thousands with a small boy's lunch (John 6:35). Later in this same chapter Jesus will use a slightly different version of this idea—not part of the primary seven "I am" statements—earning rage from the Pharisees.