Paul here quotes a proverbial saying which he uses elsewhere (see 1 Cor 5.6). The amount of yeast is very small in proportion to the total lump of dough, but it is used to make … rise (that is, to ferment or leaven) the whole lump. In the New Testament yeast is used as a symbol of the pervasive influence either of evil or of good (as in Matt 13.33). Paul’s emphasis is on the former. The meaning of the proverb is fairly obvious: evil, no matter how small it seems, will always in the end result in great harm. Paul may be applying the proverb either to the teachers, who obviously were only a handful, or to their teaching, especially to their possible insistence on circumcision as only a small thing.

The action of yeast on a batch of dough is expressed quite differently in various languages, for example, “only a little yeast can make a large batch of dough grow big,” “… can soon sour a great deal of dough,” or “… is needed for a big loaf of bread.”

The expression as they say is not in the Greek text but is legitimately added here to mark the previous statement as a proverb or popular saying. It may be rendered in some languages as “there is a saying that,” “one often hears it said,” or even “people often say.”

Quoted with permission from Arichea, Daniel C. and Nida, Eugene A. A Handbook on Paul’s Letter to the Galatians. (UBS Handbook Series). New York: UBS, 1976. For this and other handbooks for translators see here .