From Ephesians by Peter S. Williamson, reflecting on Ephesians 5:8-14
Paul explains why his readers should not be partners in evil—For you were once darkness, but now you are light in the Lord. As in chapter 2 and in 4:17–24, Paul recalls the stark contrast between his readers’ past and present conditions. In Paul’s letters, “darkness” usually refers to ignorance and spiritual or moral evil, and “light” refers to true knowledge and spiritual and moral goodness.
The Ephesians previously belonged to the darkness and were under Satan’s rule (2:1–3), the kingdom opposed to God (Col 1:13). But now they have become light “in the Lord” because they are united to Jesus, in whom truth (4:21) and divine life are found.
The fact that they have become light has implications for their conduct: Live as children of light. He repeats the catchword “walk” (NAB: “†live”) for the fourth time. “Children of light” is a †Semitic expression used by Jesus to refer to people who belong to God (Luke 16:8; John 12:36). Paul accents the fruit of their lives: for light produces every kind of goodness and righteousness and truth. Children of light manifest the kind of spiritual and moral goodness that everyone recognizes.
To clarify further how his readers should live, Paul adds, Try to learn what is pleasing to the Lord. Other versions catch the nuance of the Greek verb dokimazō better: “Try to discern” (ESV). Paul is encouraging them to reflect on what Jesus wants. Christian moral conduct is not merely spelled out for us in a set of instructions like the law of Moses. Yes, there are basic standards, like the Ten Commandments, the instructions of Eph 4:25–31 and 5:3–5, and the law of love (Rom 13:8–9; Gal 5:14). But in the many situations of life, we must discern what concretely is pleasing to the Lord. Paul does not say it explicitly here, but the indwelling Spirit of Jesus makes this discernment possible (Rom 8:2–14; Gal 5:16–23).