I have indeed inveighed sharply against impious doctrines, and I have not been slack to censure my adversaries on account, not of their bad morals, but of their impiety. And for this I am so far from being sorry that I have brought my mind to despise the judgments of men and to persevere in this vehement zeal, according to the example of Christ, who, in His zeal, calls His adversaries a generation of vipers, blind, hypocrites, and children of the devil. Paul, too, charges the sorcerer with being a child of the devil, full of all subtlety and all malice; and defames certain persons as evil workers, dogs, and deceivers. In the opinion of those delicate-eared persons, nothing could be more bitter or intemperate than Paul’s language. What can be more bitter than the words of the prophets? The ears of our generation have been made so delicate by the senseless multitude of flatterers that, as soon as we perceive that anything of ours is not approved of, we cry out that we are being bitterly assailed; and when we can repel the truth by no other pretence, we escape by attributing bitterness, impatience, intemperance, to our adversaries. What would be the use of salt if it were not pungent, or of the edge of the sword if it did not slay? Accursed is the man who does the work of the Lord deceitfully.
source: Letter of Martin Luther to Pope Leo X, Concerning Christian Liberty, September 6, 1520