He Shall Bear Witness of Me
Christ now further says:
“He shall bear witness of me."
 That is, if he is in the heart he speaks through you, and assures and confirms you in the belief that the Gospel is true. Then, as a result, the confession of the Gospel springs forth. What, then, is the Gospel? It is a witness concerning Christ, that he is God’s Son, the Saviour, and beside him there is none other. This is what Peter means when he says: “Ye are a royal priesthood, that we are elected thereto, that we preach and show forth the excellencies of Christ.” I Pet 2:9. Hence, there must always be witnessing. Witnessing loads upon itself the wrath of the whole world. Then the cross follows, then rebellions rise, then the lords and princes and all who are great become angry; for the world cannot hear, nor will it tolerate, this kind of preaching. Therefore, the Gospel is hated and spoken against.
 Reason thinks: Aye, one can, nevertheless, easily preach the Gospel in a beautifully simple and plain way, without a revolution in the world, and then it will be heartily welcomed. This is the utterance of Satan; for if I believe and say that faith in Christ alone does and accomplishes all, I overthrow the monkey play of the whole world; and that they cannot allow. Therefore, Christ’s teachings and man’s teachings cannot stand together; one must fall. Priests and monks, as they are at present, are dependent in name, character and works upon human institutions, which the Gospel thrusts to the ground. Hence, they dare not accept the Gospel, and they continue as they are.
 Thus. I say that the Christian faith is founded upon Christ alone, without anything additional. The priests will not permit their affairs and institutions to fall; in consequence seditions and rebellions follow. Therefore, there must be dissension where the Gospel and the confession of Christ are; for the Gospel opposes everything that is not of its own spirit. If the teachings of Christ and the priests were not antagonistic, they could easily stand together. They are now pitted against one another. As impossible as it is for Christ not to be Christ, so impossible is it for a monk or priest to be a Christian. Therefore, a fire must be kindled. The Lord himself, in Mt 10:34 and Lk 12:51 says: “I came not to send peace, but a sword.”
From Martin Luther’s Church Postil, Lenker Edition Vol. 3, “Day of Christ’s Ascension Into Heaven, Sermon 2.”