For ye were sometimes darkness - see the Ephesians 2:11-12 notes; 1 Corinthians 6:11 note. The meaning here is, that they were themselves formerly sunk in the same ignorance, and practiced the same abominations.
But now are ye light in the Lord - Light is the emblem of happiness, knowledge, holiness. The meaning is, that they had been enlightened by the Lord to see the evil of these practices, and that they ought, therefore, to forsake them.
Walk as children of light - see the notes on Matthew 1:1, on the use of the word “son,” or “children.” The meaning here is, that they should live as became those who had been enlightened to see the evil of sin, and the beauty of virtue and religion; compare John 12:36, where the same phrase occurs.