They are darkened in their understanding, alienated from the life of God because of the ignorance that is in them, due to their hardness of heart. (ESV)
Have you ever been in complete darkness. I mean no light at all. In a room with no windows. No crack of light coming through the door. No nightlight. Total and complete darkness. In that situation it is not possible to see the things that are around you. You could step on a lego. That would be a tragic and painful experience! But, the point being, darkness changes how we can walk. If we cannot see where we are going, we cannot go where we want to go.
Paul continues to talk about how our walk is to be different than those who are outside of the faith. He refers to them as Gentiles. He says that their understanding is darkened. This darkness has great consequences. It alienates them from the life of God. We do not like to think of people in these terms. It is hard to view a co-worker, friend, or even a family member as being in complete darkness and alienated from God because of their ignorance of him. But that is exactly how Paul describes the lives of those who have not been transformed by the grace of God. If we are to be effective in bringing people from darkness to light we must first recognize the darkness.
Now, I want you to note something Paul says in the end of this verse. He says that the ignorance that damns them in darkness is due to their hardness of heart. It implies that those in darkness have actually turned the light off so that they remain in darkness. Jesus is described in Scripture as the Light of the World. Listen to how the Gospel of John describes those in darkness and how they have reacted to the Light, which helps us to understand their self-inflicted hardness of heart.
 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 are in darkness, I invite you to allow Jesus to shine his light into your heart. He wants to forgive your sin, if you will confess it to him, and love him more than your darkness. If you are filled with His light, find someone in darkness and share the light of Jesus with them.