For ye were sometimes darkness, but now are ye light in the Lord: walk as children of light:
Sometimes - `once.' Ye ought to have no fellowship with sin, which is darkness, for your state as darkness is now PAST. Stronger than "in darkness" (Romans 2:19; Romans 6:17).
Light - not merely 'enlightened,' but enlightening others (Ephesians 5:13).
In - in union with the Lord, THE LIGHT: outside of Him all is darkness.
Children of light - not merely 'of the light;' just as "children of disobedience" on the opposite side: those whose characteristic is light. Pliny, wilting to Trajan, bears unwilling testimony to the extraordinary purity of Christians' lives contrasted with those around them.