We mock the cross when we do not take all of our guilt to it but hold some of it within, deciding how we will redeem or punish ourselves accordingly. We minimize the significance of both our sin and the cross, opting instead to hide behind a wall with our self-redemption projects or self-flagellation, and we thereby rob ourselves also of the joy and power of the cross to change us.
We mock the cross when we will not forgive someone, because we are mocking the way in which God dealt with all sin and seeks to deal with all of us, reconciling the world to Himself through the blood of Jesus. We say, "No, God. Your way is insufficient. My way is better, more just."
We mock the cross when we substitute a Christian image or ideal for God's Law. A Christian image is more attainable, and we might just convince ourselves that we are pulling it off. But there is no savior from this image, only from God's Law, so we not only empty the cross of its power but empty ourselves of any real options. Self is a very, very poor savior.