Christ says very definitely, The Holy Spirit will witness of me, of me and not of someone else. Beyond this witness of the Holy Spirit about Christ there is no sure and abiding comfort. That is why one should write the words “of me” with capital letters and diligently remember them. For of this we may be certain, that the Holy Spirit promotes no other doctrine, preaches neither Moses nor other laws whereby to comfort the conscience. If the conscience is to be comforted, it can only be 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.
Martin Luther, House Postil for the Seventh Sunday of Easter (John 15:26-16:4)