For ye were sometimes darkness
Not only dark, but darkness itself; exceeding blind, dark, and ignorant, respecting spiritual things; so the Gentiles were wont to be called by the Jews, (Kvwh) , "darkness" F11 itself; of this darkness, (See Gill on Ephesians 4:18).
But now [are ye] light in the Lord;
either in, or by the Lord Jesus Christ, the light of men, from whom all spiritual light comes; or by the Lord the Spirit, by whom the eyes of their understandings were enlightened, to see the exceeding sinfulness of sin, in heart and life; the insufficiency of their own righteousness and moral virtues, to justify them before God; and the true and right way of righteousness, life and salvation by Christ; and to have some light into the several doctrines of the Gospel, and even a glimpse of the invisible glories and realities of another world: and this light is so great, that they are not only said to be enlightened, but to be light itself; and this they have not of, and from themselves, but the Lord; and therefore should
walk as children of light;
not in sins, which are works of darkness, but in faith, truth, and holiness.