The last candle in the advent wreath symbolizes Light, and those traditions who choose to include it typically light it on Christmas Eve in preparation for the remembrance of Jesus’ birth.  
Since the moment humans first rebelled against God, we have lived in pervasive darkness.  We submitted to the base, animalistic impulses we learned from the serpent; selfishness, tribalism, arrogance, hate, violence, greed, indifference.  The image of God we were meant to bear became distorted in the shadows that we cast on creation, each other, and ourselves.  We even began to view other human beings as disposable because of our differences, claiming that the distortions in the other’s face were more grotesque than the distortions on our own.
But at the fullness of time, God saw fit to send forth His only begotten Son; begotten not made, true light of true light, the incarnate Word of God.  And for the first time since we fell, a light began to shine in the darkness which no shadow could contend with.
When that light shines on us it casts away the distortions, strips us bare of the mascarade we had grown accustomed to; it forces us to wrestle with the distortions we had once accepted as a fact of life, distortions we benefited, and even sometimes still benefit from at a cost to others.  And when we see the light held up to those we once considered grotesque and profane, the light shames our judgmental presuppositions as we glimpse God’s image in the other.  When the light shines on those we once ignored or thought beneath us, our hearts break because we see that we had been missing out on a beautiful and complex reflection of God for the shadows that surrounded them.
As Christians, when we reflect on the meaning of Christmas we must take care not to forget that by coming in the form of broken humanity, dying, and rising again Jesus fundamentally changed what it means to be human.
In Jesus’ resurrection is the beginning of the resurrection of our entire species; in Jesus, we are being made new.  There is a power to the resurrection which is active in our world now through the Church, putting to death those base animal instincts of the old way to be human, lifting up those who were crushed by those injustices, and making all things new to the glory of the God we serve.  
It’s a messy task that sometimes takes us back into places where the darkness is thick and the shadows still dance, but the light that dwells within us cannot be conquered by any amount of darkness.  
Feed the hungry, take up the case of the immigrant and refugee, give winter clothes to the poor, visit the sick and imprisoned, forgive your very politically-minded relative who disagrees with you, and find Christ in their eyes even as you continue to find the unique ways in which you bear the image of Christ yourself.  
Have a Merry Christmas.