"The Cost of Discipleship" by Dietrich Bonhoeffer is a theological work that explores the true meaning of Christian discipleship, arguing that genuine following of Jesus Christ requires a significant personal sacrifice and commitment, contrasting a "cheap grace" (easy forgiveness without life change) with a "costly grace" which demands radical transformation and obedience to Jesus' teachings, even in the face of opposition and persecution; essentially, it emphasizes that being a true disciple involves actively living out one's faith through concrete actions and ethical conduct in the world.