Just to add to Steve's excellent answer:
Jesus used the word light to highlight Christian living.
Matthew 5:14 through 16 KJV
Ye are the light of the world. A city that is set on an hill cannot be hid.Neither do men light a candle, and put it under a bushel, but on a candlestick; and it giveth light unto all that are in the house. Let your light so shine before men, that they may see your good works, and glorify your Father which is in heaven.
Since we Christians are the only image of God that the World can see, It will judge the effectiveness of the Gospel by our actions. Many times I have heard people use the bad actions of Church members as an example of why they don't need Jesus in their lives.
Here Jesus is saying that those who are his disciples need to reflect it, and Paul is just adding to that in Romans chapter 13, by saying that we should be the same when we think we are hidden by darkness as we are in the daylight.
If we get down to the bare facts of the matter God sees just as well in darkness as he does in the light of day. So it behooves us to remember that God is always watching, and act accordingly.
We should keep in mind that Jesus is at the right hand of the father making,intercessions for us; when Satan accuses us; but what can he say in our defense when we deliberately do something wrong.
Jesus is still human and I feel it is just as embarrassing to him to have to say to the father.
"Yes that is one of those I died for, so please forgive him/her."
as it is to the mother whose child throws a tantrum in a store; to say to the other mothers whose children are well behaved; "yes that's my kid."