Leaven (Yeast)
Leaven is the household ferment that turns dough — and Scripture takes it up at two registers. In the kitchen it is the agent that raises bread; on the altar and at Passover it is the substance Israel must put away; in the prophets and in the New Testament it becomes a figure for what permeates from a small starting point into a whole lump, whether for ill or for good.
Bread and the Baker's Trade
Leaven appears first as the ordinary baker's substance. Israel's haste at the Exodus is measured against it: "the people took their dough before it was leavened, their kneading-troughs being bound up in their clothes on their shoulders" (Ex 12:34), and the bread that resulted "was not leavened, because they were thrust out of Egypt, and could not tarry, neither had they prepared for themselves any victuals" (Ex 12:39). Hosea takes the same domestic process and turns it against Israel: "they are as an oven heated by the baker; he ceases to stir [the fire], from the kneading of the dough, until it is leavened" (Ho 7:4).
Leaven in the Offerings
Within the worship system, leaven is admitted in some offerings and barred from others. With the peace-offering for thanksgiving, leavened cakes are brought: "With cakes of leavened bread he will offer his oblation with the sacrifice of his peace-offerings for thanksgiving" (Le 7:13); Amos echoes the same thanksgiving setting — "offer a sacrifice of thanksgiving of that which is leavened, and proclaim freewill-offerings and publish them: for this pleases you⁺, O you⁺ sons of Israel, says the Sovereign Yahweh" (Am 4:5). The wave-offering of the firstfruits at Pentecost is likewise leavened: "From your⁺ habitations you⁺ will bring bread as a wave offering: two [loaves] of two tenth parts [of an ephah]: they will be of fine flour, they will be baked with leaven, for first fruits to Yahweh" (Le 23:17).
Against these admissions stand explicit bars. The meal-offering excludes leaven both from its composition and from the altar fire: "No meal-offering, which you⁺ will offer to Yahweh, will be made with leaven; for you⁺ will burn no leaven, nor any honey, as an offering made by fire to Yahweh" (Le 2:11). The priest's portion of the meal-offering carries the same restriction — "It will not be baked with leaven. I have given it as their portion of my offerings made by fire; it is most holy, as the sin-offering, and as the trespass-offering" (Le 6:17) — and Moses directs Aaron to "eat it without leaven beside the altar; for it is most holy" (Le 10:12). Two parallel rules tie the bar to the blood of the sacrifice: "You will not offer the blood of my sacrifice with leavened bread; neither will the fat of my feast remain all night until the morning" (Ex 23:18); "You will not offer the blood of my sacrifice with leavened bread; neither will the sacrifice of the feast of the Passover be left to the morning" (Ex 34:25).
Passover and the Seven-Day Purge
At Passover the bar is total and household-wide. "Seven days you⁺ will eat unleavened bread; even the first day you⁺ will put away leaven out of your⁺ houses: for whoever eats leavened bread from the first day until the seventh day, that soul will be cut off from Israel" (Ex 12:15). The same seven-day window forbids any leaven from being found, and joins native and sojourner under one rule: "Seven days there will be no leaven found in your⁺ houses: for whoever eats that which is leavened, that soul will be cut off from the congregation of Israel, whether he is a sojourner, or one who is born in the land" (Ex 12:19). The eating-clause is sealed twice over: "You⁺ will eat nothing leavened; in all your⁺ habitations you⁺ will eat unleavened bread" (Ex 12:20).
The Exodus-day memorial repeats the rule and dates the feast. "Remember this day, in which you⁺ came out from Egypt, out of the house of slaves; for by strength of hand Yahweh brought you⁺ out from this place: there will be no leavened bread eaten" (Ex 13:3). "This day you⁺ go forth in the month Abib" (Ex 13:4). And the seven-day window is restated with both leavened bread and leaven itself absent from sight: "Unleavened bread will be eaten throughout the seven days; and there will be no leavened bread seen with you, neither will there be leaven seen with you, in all your borders" (Ex 13:7). Deuteronomy widens the scope from the house to the whole land and ties it to the paschal flesh: "there will be no leaven seen with you in all your borders seven days; neither will any of the flesh, which you sacrifice the first day at evening, remain all night until the morning" (De 16:4).
A Symbol for Malice and Wickedness
Paul takes the Passover purge and reads the church through it. "Don't you⁺ know that a little leaven leavens the whole lump? Purge out the old leaven, that you⁺ may be a new lump, even as you⁺ are unleavened. For our Passover also has been sacrificed, [even] Christ: therefore let us keep the feast, not with old leaven, neither with the leaven of malice and wickedness, but with the unleavened bread of sincerity and truth" (1 Cor 5:6-8). The same proverb returns to Galatia in the same words: "A little leaven leavens the whole lump" (Ga 5:9).
The Leaven of the Pharisees
In the Gospels Jesus turns the figure on his contemporaries. To the disciples in the boat: "Take heed, beware of the leaven of the Pharisees and the leaven of Herod" (Mr 8:15). To the gathered multitudes Luke gives the explicit gloss: "Take heed to yourselves [and stay away] from the leaven which is the hypocrisy of the Pharisees" (Lu 12:1).
The Parable of the Hidden Leaven
A second figurative use is positive. In Luke's version of the kingdom parable the leaven becomes the likeness of the kingdom's quiet, total work: "It is like leaven, which a woman took and hid in three measures of meal, until it was all leavened" (Lu 13:21). The agent, the hiding, the measured quantity, and the full permeation are each named, so the leaven's distinguishing trait — thorough penetration through the whole batch — carries the image whether the lump is the church to be purged (1 Cor 5; Ga 5:9) or the meal to be wholly leavened (Lu 13:21).