The words that Paul here uses are imagery common to the Bible, common to church history. The darkness of sin is often spoke of, the refusal to believe. It is contrasted to the light of the gospel; walking with the Lord is to walk in the light. The world outside of Christ very early in the history of the church was seen as the dark age. Now, we are familiar with that term in its anti-Christian use by modern scholars to describe the world after the fall of Rome, and with the triumph of Christianity.