This is what we mean by cheap grace, the grace which amounts to the justification of sin without the justification of the repentant 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 contrition. Cheap grace is grace without discipleship, grace without the Cross, grace without Jesus Christ, living and incarnate.
Costly grace is the treasure hidden in the field; for the sake of it a man will gladly go and sell all that he has. It is the pearl of great price to buy which the merchant will sell all his goods. It is the kingly rule of Christ, for whose sake a man will pluck out the eye which causes him to stumble, it is the call of Jesus Christ at which the disciple leaves his nets and follows Him.
Costly grace is the gospel which must be sought again and again, the gift which must be asked for, the door at which a man must knock.
Such grace is costly because it calls us to follow, and it is grace because it calls us to follow Jesus Christ. It is costly because it costs a man his life, and it is grace because it gives a man the only true life. It is costly because it condemns sin and grace because it justifies the sinner. Above all, it is costly because it cost God the life of His son: ‘ye were bought at a price,’ and what has cost God much cannot be cheap for us. Above all, it is grace because God did not reckon His Son too dear a price to pay for our life, but delivered Him up for us. Costly grace is the Incarnation of God.
–Dietrich Bonhoeffer, The Cost of Discipleship