Martin Luther preached these beautiful words in 1532. They are based on John 15:26-16:4, specifically Jesus’ words, “He [the Holy Spirit, the Comforter] will bear witness of me.”
“If the conscience is to be comforted, it can only be comforted by the preaching of Christ’s death and resurrection – this alone comforts. In contrast, all other preaching of law, good works, holy living, whether commanded by God or men, is incapable of comforting a person in times of need and death; instead it leaves him uncertain and in despair, frightened and tormented. If we consider God without Christ, we find no comfort but only righteous wrath and displeasure. But whoever preaches Christ proclaims and brings true comfort, so that it will be impossible for hearts not to be joyous and of good cheer.”
“…No one in need of comfort, therefore, should wait until the Holy Spirit in all his majesty speaks to him personally from heaven. For the Holy Spirit carries out his witness, publicly in the sermon. This is where you must seek and await him, til the word which you hear with your ears witnesses inwardly of Christ in your heart. But such inward witness does not come about until the external, spoken witness of the Word is heard which tells us that Christ became man, was crucified, died, and rose again for our sakes.”
These quotes are from pages 148-149 of Luther’s Sermons (the Baker edition), volume 6.