BY DIETRICH BONHOEFFER
Cheap grace means grace as a doctrine, a principle, a system. It means forgiveness of sins proclaimed as a general truth, the love of God taught as the Christian "conception" of God. An intellectual assent to that idea is held to be of itself sufficient to secure remission of sins...
[Costly grace] 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" (1 Cor. 6:20), and what has cost God much cannot be cheap for us