for at one time you were darkness, but now you are light in the Lord. Walk as children of light (ESV)
The Apostle Paul is a master at using various illustrations to help us to understand what it means to be a Christian. I believe that Christians always fight a culture that attempts to blur the lines between Christian and Non-Christian. The more that a Christian acts in ways that are holy and pure the more the world feels that their sinfulness is exposed. There is a reason for that!
Paul talks about what we were, what we are now, and how we should live moving forward. All of this is so important for us. If we don't know exactly what God has done in our lives it will be hard to walk in ways that please him.
At one time we were darkness. All of us. That darkness is a description of our spiritual condition. Our sinful nature places us in darkness. In fact, all of those who do not know Jesus Christ are filled with darkness and following the power of darkness. In Ephesians 6:12 this darkness is described as being ruled by dark powers:
 For we do not wrestle against flesh and blood, but against the rulers, against the authorities, against the cosmic powers over this present darkness, against the spiritual forces of evil in the heavenly places. (ESV)
These powers are at work in the hearts of men and women. They are active in keeping us in darkness. They fight against the Lord, who is full of light. You would think that those in darkness would hate being in darkness. But, the fact is that because those who are in darkness are deceived they like to continue in their deception. Light exposes their sinful darkness, so they choose to stay in the dark lest their sin be exposed.
 And this is the judgment: the light has come into the world, and people loved the darkness rather than the light because their works were evil.  For everyone who does wicked things hates the light and does not come to the light, lest his works should be exposed.(ESV)
If you belong to Jesus it is because of a supernatural encounter with God, who is the Light. Don't forget where you came from. And ask Jesus to help you to bring the light of Christ to those who remain in darkness.