Yeast

“A little leaven leavens the whole lump.”

— Galatians 5:9 (ESV)

Yeast, called “leaven” throughout Scripture, figures pervasive influence: a small amount works through a whole batch of dough until it affects all of it, the way sin, corrupted teaching, or hypocrisy can spread through a person or a congregation from one source. Most biblical uses carry this negative sense. One parable of Jesus applies the same image to the growth of the kingdom of God instead. Israel’s Passover required unleavened bread, which the New Testament reads as pointing to the removal of sin since Christ, the true Passover lamb, has been sacrificed.

Biblical Usage

Old Testament: The Passover and Unleavened Bread

When the Lord instituted the Passover in Egypt, the lamb was to be eaten “with unleavened bread and bitter herbs” in haste, and for seven days afterward no leaven was to be found in an Israelite home (Exod. 12:8, 15, 19-20). Herman Bavinck traces the ritual’s details, roasted whole and eaten in haste, directly into its New Testament fulfillment: Paul “expressly states in 1 Cor. 5:7 that our paschal lamb, Christ, has been slain, and that believers must therefore cleanse out the old leaven of sin and walk as ‘unleavened’ persons, new creatures, unmixed with any unrighteousness.”¹ Paul draws the same connection himself, calling believers to keep the feast “not with the old leaven, the leaven of malice and evil, but with the unleavened bread of sincerity and truth” (1 Cor. 5:7-8).

New Testament: A Symbol of Corrupting Influence

Most New Testament uses of leaven carry the negative sense already present in the Passover law. Jesus warns his disciples three times to beware “the leaven of the Pharisees” (Matt. 16:6; Mark 8:15; Luke 12:1), and each Gospel pairs it with a different further danger: Matthew adds the leaven “of the Sadducees” (Matt. 16:6), Mark adds “the leaven of Herod” (Mark 8:15), and Luke supplies its own gloss, “which is hypocrisy” (Luke 12:1). Reflecting on this warning, John Calvin reads the leaven of the Pharisees as the corruption of true doctrine: Christ, he writes, “restored [the moral law] to its integrity by maintaining and purifying it when obscured by the falsehood, and defiled by the leaven of the Pharisees.”² Thomas Boston likewise names “the leaven of hypocrisy, Luke xii. 1,” as one mark of an unregenerate profession of religion.³

Paul applies almost the same words to two further dangers. Confronting the Corinthian church’s toleration of a member’s unrepented sexual sin, he asks, “Do you not know that a little leaven leavens the whole lump?” (1 Cor. 5:6), and commands the church to expel the man rather than let his sin spread unchecked. Calvin cites this verse directly for one purpose of church discipline, “that the good may not… be corrupted by constant communication with the wicked.”⁴ Alexander Strauch draws the same lesson from the same failure: “one man’s sexual sin,” he writes, “contaminated the whole assembly.”⁵ Writing to the Galatians about a different threat, teachers pressing circumcision and law-keeping on Gentile believers, Paul repeats the warning almost verbatim: “A little leaven leavens the whole lump” (Gal. 5:9). There the leaven is false teaching rather than tolerated sin, but the image is the same: something small, once let in, works through the whole.

New Testament: The Parable of the Leaven

Jesus also compares the kingdom of heaven itself to leaven: “The kingdom of heaven is like leaven that a woman took and hid in three measures of flour, till it was all leavened” (Matt. 13:33; Luke 13:20-21). Augustus H. Strong reads this parable’s leaven differently from every other New Testament use, pairing it instead with the parable of the mustard seed just before it: where the mustard seed illustrates the kingdom’s outward, extensive growth, he argues, “the parable of the leaven… apparently illustrates the intensive… development of the kingdom of God,” and “it is as impossible to confine the reference of the leaven to the spread of evil as it is impossible to confine the reference of the mustard seed to the spread of good.”⁶ On this reading, the image pictures pervasive, working-through growth, not the corruption Jesus’s other use of the same figure describes.

Related Doctrines

References

Herman Bavinck, Reformed Dogmatics: Abridged in One Volume, ed. John Bolt (Grand Rapids: Baker Academic, 2011), chap. 22, “The Spirit’s Means of Grace.” ↩
John Calvin, Institutes of the Christian Religion, trans. Henry Beveridge (Edinburgh: Calvin Translation Society, 1845-1846), 2.8.7. ↩
Thomas Boston, Human Nature in Its Fourfold State (Printed for the booksellers, 1787), 129-213. ↩
Calvin, Institutes, 4.12.5. ↩
Alexander Strauch, Biblical Eldership: Restoring the Eldership to Its Rightful Place in the Local Church, 4th ed. (Littleton, CO: Lewis and Roth Publishers, 2017), chap. 28, “Understanding the Relationship between the Elders and the Congregation,” 285-302. ↩
Augustus Hopkins Strong, Systematic Theology: A Compendium and Commonplace-Book Designed for the Use of Theological Students, vol. 3 (Philadelphia: The Griffith & Rowland Press, 1909). ↩