“A little yeast works through the whole batch of dough” (Galatians 5:9 NIV).
A good baker knows that the addition of yeast to a batch of dough can help produce light, airy pastries, cakes, breads, and other types of baked goods. But even though this ingredient helps produce the taste and aroma associated with a freshly- baked loaf of bread, it assumes a very different role when used as a Biblical metaphor.
You see, yeast is often used in the Bible as an illustration for sin. Just as a small amount of yeast (or leaven) will cause a piece of dough to rise, a small amount of sin can produce a significant (and detrimental) effect. This metaphor was not lost upon Paul the Apostle, for he used a related example in his letter to the church at Corinth: “Your boasting is not good. Don’t you know that a little leaven leavens the whole batch of dough? Clean out the old leaven so that you may be a new unleavened batch, as indeed you are…” (CSB).
Jesus also used a similar illustration in speaking with His disciples: “Be on your guard against the yeast of the Pharisees, which is hypocrisy” (Luke 12:1 NIV). In the New Testament gospel of Mark, Jesus expanded this cautionary message beyond the spiritual leadership of His day to include governmental rulers as well: “…Watch out! Beware of the yeast of the Pharisees and the yeast of Herod” (Mark 8:15 HCSB).
Much like the effect that yeast exerts upon a lump of dough, an attitude of hypocrisy had spread among the religious leadership of first-century Israel. On the other hand, a lust for power, an attitude of compromise, and an irreligious mindset permeated the Herodian political dynasty of that era. If Jesus’ followers permitted themselves to be influenced by such behaviors, they were sure to be adversely affected by them.
So the idea behind this passage is simple: it only takes a small amount of something harmful to negatively affect something much larger. Today we might adapt this concept beyond the false teachers of first-century Galatia to other areas such as alcohol abuse, pornography, drug use, or gambling to name a few.
No one ever began their involvement in such things with the intent to become an alcoholic, a drug abuser, or a compulsive gambler. But just like the damaging influence exerted by the false teachers of Galatia, a small amount of activity in these areas might eventually lead to something far more destructive.