Before my youngest son was old enough to say "sorry," he expressed remorse by laying his head against the person he had wronged. Sweet right?
Maybe not. After he learned he could get away with anything by melting hearts in the aftermath of his carnage, things got rough for a while. For example, he would smack my wife and then immediately lay his head against her for his free pass. He had learned Romans 5:20 ("where sin abounded, grace did much more abound"). But the grace of Romans 5:20 can be pernicious in isolation because it can lead to a doctrine of cheap grace.
So one day when it happened again, I (metaphorically) preached the very next verse (Romans 6:1) to him. I said, "I love you and will always forgive you, but shall we continue in sin that grace may abound? God forbid. How shall we who are dead to sin live any longer therein?" I'm pretty sure he did not understand at the time.
In that respect he is like a lot of adults I have talked to over the years. I remember one case in particular where a friend actually told me that he could do anything he wanted because God's grace was sufficient to cover his sins. That man went on to experience many heartaches, including divorce and the alienation of his children. Yes, God's grace is sufficient to cover all of our sins. But sin nevertheless has temporal consequences, including broken relationships. Cheap grace hurts.
I can learn from my own preaching. Have I used "grace covers all sin" as a cheap get-out-of-jail-free card? If I have, I must repent. Grace is not cheap. It came at an enormous cost -- God himself becoming flesh and bleeding and dying on Calvary. As Paul said to the Corinthians, "I am bought with a price." A very costly price. Christ has paid the penalty for my sin; may it never be that I treat his staggering act of redemptive grace as a cheap talisman. Instead, may I always remember the enormous price he paid for me and never treat his grace lightly.
One reason we can't sin that grace may abound is that we are in time, even if God is not.
To traffic in cheap grace denies yourself and others the sheer awestruck wonder of real grace. I suspect an addiction to cheap grace hurts more than individuals. It hurts churches, communities, and nations.