sometimes - “once.” The emphasis is on “were.” Ye ought to have no fellowship with sin, which is darkness, for your state as darkness is now PAST. Stronger than “in darkness” (Rom 2:19).
light - not merely “enlightened”; but light enlightening others (Eph 5:13).
in - in union with the Lord, who is THE LIGHT.
children of light - not merely “of the light”; just as “children of disobedience” is used on the opposite side; those whose distinguishing characteristic is light. Pliny, a heathen writing to Trajan, bears unwilling testimony to the extraordinary purity of Christians’ lives, contrasted with the people around them.