Rev. Kuiper is pastor of Southeast Protestant Reformed Church in Grand Rapids, Michigan.
Leaven is yeast, a substance made of one-celled, microscopic plants which multiply rapidly in the presence of heat and moisture. When yeast reacts with flour it produces a gas (CO2) which causes the dough to rise; when the dough is kneaded, the bubbles of gas are broken down so that the resulting bread is of uniform texture and lightness. Leaven, given time, works through the entire mass of dough so that no part is left untouched. “Leaven leaveneth the whole lump” (I Cor. 5:6). In Scripture, with one exception, leaven is given a negative, sinful connotation; we are warned against this leaven sharply.
The one exception is the parable of Jesus (Matt. 13:33;Luke 13:21) in which He likens the kingdom of heaven to leaven which a woman places in three measures of flour. The invisible power of the kingdom is the Holy Spirit working with the preached Word. As leaven, the preaching takes time to do its work. As leaven, the preaching penetrates into every part of the church and her membership, bringing even their thoughts into captivity to Christ. We are called to be patient in this regard. As you cannot see leaven doing its work throughout the lump, so we do not always see fruit upon the preaching, at least not as much or as quickly as we would like. But the work is sure and the kingdom is safe.
For the rest, the biblical teaching regarding leaven is of a warning nature. Jesus rages against the leaven of the Pharisees which is hypocrisy (Luke 12:1), against the leaven of the Pharisees and the Sadducees which is false doctrine (Matt. 16:6), and against the leaven of the Herodians which is worldly compromise (Mark 8:15). Add to this Paul’s warning, “A little leaven leaveneth the whole lump” (Gal. 5:9) and we see that false doctrine, be it ever so small and innocent seeming, will work its way through the whole body of doctrine until it is all thoroughly corrupted. If justification is by circumcision or any human work, Christ shall profit you nothing. Our works are not the whole, or the smallest part, of our righteousness before God.
Not only does false doctrine work as leaven, but this is true of sin as well. The burden of Paul in I Corinthians 5 is to show that a lack of discipline in the church will have two perilous results: 1) Sin unconfessed and unforsaken (here fornication) will work its way through the impenitent’s life so that in time all sins are practiced and no sin is abhorred or confessed, and 2) Sin not rooted out by discipline will spread as leaven throughout the congregation, so that the entire congregation comes under the wrath of God. Specifically, Paul calls leaven malice and wickedness, while being unleavened spiritually is sincerity and truth (v. 8).
The New Testament teaching on leaven is based squarely on the Feast of Unleavened Bread that was instituted when Israel was delivered out of Egypt (Ex. 12:15-20). During the seven days following the Passover Feast the Israelites might not eat leavened bread or have any leaven in their houses. One reason for this was the haste required (v. 11), and the fact that they were thrust out of Egypt and could not tarry (v. 39). But the reason goes deeper. Unleavened bread pointed to holiness of heart and conduct which becomes the ransomed people of God. Sincerity and truth ought to characterize all their behavior. Symbolically, this taught what is clearly revealed in the New Testament, that Christ, our Passover, was sacrificed to redeem unto Himself a people who put off the old man and his sins and are made new creatures in Christ (I Cor. 5:7).