Martin Luther, from his Commentary on Galatians:
No man can say that we are seeking the favor and praise of men with our doctrine. We teach that all men are naturally depraved. We condemn man’s free will, his strength, wisdom, and righteousness. We say that we obtain grace by the free mercy of God alone for Christ’s sake. This is no preaching to please men. This sort of preaching procures for us the hatred and disfavor of the world, persecutions, excommunications, murders, and curses.
Can’t you see that I seek no man’s favor by my doctrine? asks Paul. If I were anxious for the favor of men I would flatter them. But what do I do? I condemn their works. I teach things only that I have been commanded to teach from above. For that I bring down upon my head the wrath of Jews and Gentiles. My doctrine must be right. It must be divine. Any other doctrine cannot be better than mine. Any other doctrine must be false and wicked.
With Paul we boldly pronounce a curse upon every doctrine that does not agree with ours. We do not preach for the praise of men, or the favor of princes. We preach for the favor of God alone whose grace and mercy we proclaim. Whosoever teaches a gospel contrary to ours, or different from ours, let us be bold to say that he is sent of the devil.