Yeast is often used as a metaphor in the New Testament:
Matthew 16:6 (NIV):
"Be careful," Jesus said to them. "Be on your guard against the yeast of the Pharisees and Sadducees."
For us this command is puzzling. Why would anyone think yeast is bad? If he'd said poison, sure- we'd get that, but by saying yeast he even confused his own followers. A Rabbi on the other hand, would have understood:
The Rabbis, in speaking of the evil desire ("yeẓer ha-ra'"), called it "the leaven that is in the dough"
As in the following passage:
1 Corinthians 5:8 (NIV):
Therefore let us keep the Festival, not with the old yeast, the yeast of malice and wickedness, but with bread without yeast, the bread of sincerity and truth.
To us yeast is something we buy in a packet so the full meaning of this is lost on us. So let's look into it. Jewishencyclopedia says:
The ordinary bread consisted of dough mixed with fermented dough, which raised the mass into soured bread, while in the kneading trough.
What is fermented dough? Wikipedia says:
Researchers speculate that a mixture of flour meal and water was left longer than usual on a warm day and the yeasts that occur in natural contaminants of the flour caused it to ferment before baking.
This is confirmed by a passage in the Talmud:
Talmud - *Mas. Menachoth 52b footnote 17:
a little flour is taken from the meal-offering, is mixed with water and is allowed to stand for some time until it becomes leavened, and this serves as yeast for leavening the rest of the meal-offering.
So people would have been able to see the difference between dough which hadn't risen, and dough that had. Yeast was an impurity that made bread rise and impurity is what causes people to sin. That's how yeast and sin came to be associated with each other. So let's look at the following passages in that light:
Matthew 16:12 (NIV):
Then they understood that he was not telling them to guard against the yeast used in bread, but against the teaching of the Pharisees and Sadducees.
So what he meant was the false teachings that were not originally part of Judaism.
Luke 12:1 (NIV):
Meanwhile, when a crowd of many thousands had gathered, so that they were trampling on one another, Jesus began to speak first to his disciples, saying: "Be on your guard against the yeast of the Pharisees, which is hypocrisy.
So their good teachings were combined with them being a bad example, which ruined the whole lesson.
1 Corinthians 5:6-7 (NIV):
Your boasting is not good. Don't you know that a little yeast works through the whole batch of dough Get rid of the old yeast that you may be a new batch without yeast—as you really are.
If you boast, others will too and in the end everyone will be puffed up.This is bad. Look at yourself honestly and see that you don't really have a reason to boast.
So why were Jesus followers confused by the metaphor? Well, maybe such things just went over their heads or maybe it was that, being single men, they just did what we do- buy our bread from the store:
Mark 6:37 (NIV):
But he answered, "You give them something to eat." They said to him, "That would take eight months of a man's wages ! Are we to go and spend that much on bread and give it to them to eat?"
Hence our mutual confusion.
* Starred passages are from the Talmud. They are from The Soncino Talmud (©1973 Judaica Press, Inc. and ©1965, 1967, 1977, 1983, 1984, 1987, 1988, & 1990 Soncino Press, Ltd.)
All NIV Bible quotes are from:
New International Version (NIV)
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taken from http://www.biblegateway.com