We are no longer slaves to sin. We are liberated IF we do what is required of us. The Sunday lesson teaches us what we need to do. Most of all, we are not to show off, boast or brag about our fasting. Fasting is always meant to be private - between us and God.
Sin wins when it causes division, opposition, separation and hatred.
The weapon against all of these is forgiveness. By way of it we re-establish unity, harmony, strength and love. When we forgive we put God between us and our enemies. When we forgive, the renewal of strained relationships is not only possible but guaranteed. Forgiveness is a total act of God's great love.