Horatius Bonar was a prolific and hugely popular author. This is what he wrote of Luther:
When first awakened to a sense of sin, Luther became unspeakably troubled. Once and again deep anguish took hold of his soul, and it seemed as if he would sink under it. On one occasion he had
been conversing with a friend upon the things of God. No sooner had the conversation ended, than the truths of which they had been speaking struck home with awful power to the tossed soul of Luther. He left the room and sought the nearest chamber to give vent to the feelings of his bursting heart. He threw himself upon the bed and prayed aloud in agony; repeating over and over again these words of the apostle, “He hath shut them all up in unbelief, that He might have mercy upon all” (Rom. 11:32).