“Once you were darkness” suggests ignorance. No one sees clearly in shadows. The ignorance referred to is disbelief, harking back to pagan days. Those who lived in darkness have been drawn to faith by the grace of God, which has been brought to light, that is, truth. Things that exist are made apparent in the light. Paganism remains deluded as with covered eyes they celebrate their own mysteries as if in the shadows of a cave.
Note the balanced way in which he exhorts them. First he approaches them from the viewpoint of Christ, saying “love one another and do no wrong to anyone.” Then he approaches them from the viewpoint of their punishment and Gehenna … reminding them of their previous wickedness, as if to say “remember what you were and what you have become.” .
Observe how wisely he urges them forward; first, from the thought of Christ, that you love one another, and do injury to no man; then, on the other hand, from the thought of punishment and hell-fire. For you were once darkness, says he, but are now light in the Lord. Which is what he says also in the Epistle to the Romans; What fruit then had ye at that time in the things whereof you are now ashamed? Romans 6:21, and reminds them of their former wickedness. That is to say, thinking what ye once were, and what you are now become, do not run back into your former wickedness, nor do despite to the grace Hebrews 10:29 of God.
