"I am the light of the world." Lighting up the world is our function. The Course is teaching us to remember Who we are, and to begin to live as Who we are. We are lights, and we can live as lights in this world, through our forgiveness sharing the happy news of freedom from all guilt.
As St. Francis of Assisi prayed, "Lord, make me an instrument of Thy peace." May I leave everyone I meet a little brighter today. May the world seem a little less dark for everyone I encounter. May each one I touch feel more loveable as a result of meeting me. May I ask to see the light in every situation; may I respond to darkness with light.
"Forgiveness is my function as the light of the world." If I do not feel like the light of the world this day, let me forgive another; everyone I forgive will show me the light in myself. It's even okay that I don't yet understand what true forgiveness really is; that can't stop me if I am willing to learn, and I am willing. Every situation that seems to bring distress is a chance to learn what forgiveness really is. I don't want to use the circumstances of my day for any purpose other than God's. Let everything be grist for the mill.