"This entire Epistle gives ample evidence of how disappointed Paul was over the fall of the Galatians and of how often he pounded at them — now with reproof, now with appeals — about the very great and inestimable evils that would follow their fall unless they reconsidered. This care and admonition, so fatherly and truly apostolic, had no effect at all on some of them; for very many of them no longer acknowledged Paul as their teacher but vastly preferred the false apostles, from whom they imagined that they had derived true doctrine rather than from Paul.
"Finally the false apostles undoubtedly slandered Paul among the Galatians in this way: Paul, they said, was a stubborn and quarrelsome man, who was shattering the harmony among the churches on account of some trifle, for no other reason than because he alone wanted to be right and to be praised. With this false accusation they made Paul detestable in the eyes of many. Others, who had not yet fallen completely away from Paul’s teaching, imagined that there was no harm in disagreeing a little with him on the doctrines of justification and faith. Accordingly, when they heard Paul placing such great emphasis on what seemed to them a matter of such minor importance, they were amazed and thought: ‘Granted that we have diverged somewhat from Paul’s teaching and that there is some fault on our side, still it is a minor matter. Therefore he should overlook it or at least not place such great emphasis on it. Otherwise he could shatter the harmony among the churches with this unimportant issue.’
"Paul answers them with this excellent proverbial statement: ‘A little yeast leavens the whole lump’ (Galatians 5:9). This is a caution which Paul emphasizes. We, too, should emphasize it in our time. For the sectarians who deny the bodily presence of Christ in the Lord’s Supper accuse us today of being quarrelsome, harsh, and intractable, because, as they say, we shatter love and harmony among the churches on account of the single doctrine about the Sacrament. They say that we should not make so much of this little doctrine, which is not a sure thing anyway and was not specified in sufficient detail by the Apostles, that solely on its account we refuse to pay attention to the sum total of Christian doctrine and to general harmony among all the churches. This is especially so because they agree with us on other articles of Christian doctrine. With this very plausible argument they not only make us unpopular among their own followers; but they even subvert many good men, who suppose that we disagree with them because of sheer stubbornness or some other personal feeling. But these are tricks of the devil, by which he is trying to overthrow not only this article of faith but all Christian doctrine.
"To this argument of theirs we reply with Paul: ‘A little yeast leavens the whole lump.’ In philosophy a tiny error in the beginning is very great at the end. Thus in theology a tiny error overthrows the whole teaching. Therefore doctrine and life should be distinguished as sharply as possible. Doctrine belongs to God, not to us; and we are called only as its ministers. Therefore we cannot give up or change even one dot of it (Matt. 5:18). Life belongs to us; therefore when it comes to this, there is nothing that the Sacramentarians can demand of us that we are not willing and obliged to undertake, condone, and tolerate, with the exception of doctrine and faith, about which we always say what Paul says: ‘A little yeast, etc.’ On this score we cannot yield even a hairbreadth. For doctrine is like a mathematical point. Therefore it cannot be divided; that is, it cannot stand either subtraction or addition. On the other hand, life is like a physical point. Therefore it can always be divided and can always yield something." (Luther’s Works, Volume 27, CPH 1963)