Note 15 at Eph. 5:8: In the previous verse, Paul had admonished the Ephesians not to become partakers with unbelievers in their sins. Why? What was the motive for living separate? He gives that motive in this verse. It is because our nature has been changed. We were darkness, now we are light (see note 16 at this verse).

Many people argue for holiness in order to obtain relationship with God. Paul is advocating holiness because of the relationship with God that we already have. It's the nature of a Christian to walk in the light and not in darkness. If Christians were rightly informed of who they are and what they have in Christ, holiness would just naturally flow out of them. It's their nature.

Note 16 at Eph. 5:8: Paul did not say that we were getting more and more light from the Lord, but that we are, right now, light in the Lord. A true believer becomes light (signifying all the goodness of God versus darkness, which is the evil of the devil) the moment he is born again in his inner man. That is a positional truth that doesn't fluctuate with our performance. But it needs to be more than just a positional truth. It needs to become an experiential reality in our physical lives.

That's what Paul is saying here. Before we were born again, we were not just part time children of the devil. We were lost all the time, even when we were acting good. Now we have become full time children of light. That's who we are. That's our nature. It needs to become our experience.