The essence of grace, we suppose, is that the account has been paid in advance [at the Cross]; and, because it has been paid, everything can be had for nothing. Since the cost [of Christ's death] was infinite, the possibilities of using and spending it are infinite. What would grace be if it were not cheap?
He also goes on to define this "cheap grace":
[Cheap grace is] the grace which amounts to the justification of sin without the justification of the sinner who departs from sin and from whom sin departs. Cheap grace is not the kind of forgiveness of sin which frees us from the toils of sin. Cheap grace is the grace we bestow on ourselves.
Cheap grace is the preaching of forgiveness without requiring repentance, baptism without church discipline, Communion without confession, absolution without personal confession. Cheap grace is grace without discipleship, grace without the cross, grace without Jesus Christ, living and incarnate.
I think that viewing justification as God's acceptance of sin instead of the acceptance of the sinner is so insightful. It's so easy for me to think, "Oh, I'm covered by grace; I'm forgiven; I'm set right with God." That's true--I am right before God through faith in Jesus Christ. But my sins never were nor ever will be right. Salvation is eminently personal: I am in fellowship with Christ. But my sins never are. My sins may be expiated, expunged, or propitiated; but they are never justified.