The history of the Church , too, illustrates the importance of understanding this distinction. Corruption entered the Church when Law and Gospel began to be mingled. A review of the writings of the Church Fathers soon reveals the cause of the Church’s misery in those early days: people did not know how to distinguish properly between Law and Gospel. Until the sixth-century we still find glorious testimonies regarding this distinction, but from that time we notice that this light starts to grow dim and that the distinction is gradually forgotten.
Law & Gospel: How to Read and Apply the Bible by C. F. W. Walther (A Reader’s Edition) p 70