Most of the time, commentators interpret this parable just as they interpret the Parable of the Mustard Seed—that the Kingdom would grow big and eventually encompass the whole earth, and everything would be great. Hallelujah! But is this correct?
When the Jews heard this parable, they must have been astounded. If Jesus told us that the Kingdom of God was like leaven in bread, what would we think? It does not sound very good to us—nor did it sound right to the Jews—because we know what leaven represents in Scripture: the corruption of sin. How can the Kingdom be likened to leaven? It is almost unthinkable that the Kingdom of God would be full of leaven throughout. Is the Kingdom evil? Is it full of sin? This does not square with what we learn in the Old Testament. The Kingdom is supposed to be glorious and pure, and Jesus is telling us that the Kingdom is full of leaven. How can this be?
And we are right! Everywhere else in the Bible where the word "leaven" or "unleavened" appears, "leaven" carries with it a negative implication. Yet, according to the commentators, this one case is the exception! In 87 out of 88 times, it means something bad, but here in Matthew 13, leaven is positive. Why? It does not make sense for a God who is the same yesterday, today, and forever (Hebrews 13:8). Leaven must still be negative here.
The commentators are uncomfortable with the idea that the Kingdom of God in its present form can have leaven in it, that it could be full of sin. But we need to remember that Jesus was seeing what would happen between the time He died and the time He returned. He saw that the people would be full of leaven, and they would always be, until they were changed to spirit.
That is the beauty of grace—that while we were yet sinners, Christ died for us, and we can then come under His blood and be cleaned. This does not mean we are clean forever—we still sin after we are cleaned. So we have to go back before the throne of grace and plead for mercy and forgiveness again and again and again—even up until the time that we die or we are changed. We sin because we are full of leaven, and we spend our whole lives getting rid of it.
Every year, we keep the Days of Unleavened Bread to depict just this process and to be thankful that we have this sacrifice—Christ our Passover—who saves us and forgives us. In the Levitical sacrifices, no leaven could be in any of the offerings that were made (Leviticus 2:11), because they typified the sinless Christ. The two wave loaves that were offered on the day of Pentecost (Leviticus 23) were made with leaven, because they represent us, the Old Testament and the New Testament, or the Old Covenant and the New Covenant—the churches of those times that were full of leaven, that is, sinful people. But God accepts them because the blood of Christ cleanses us from all sin (I John 1:7). He knows our frame and gives us grace (Psalm 103:14).
In I Corinthians 5:6, Paul writes, "Your glorying is not good. Do you not know that a little leaven leavens the whole lump?" This sounds like the Parable of the Leaven. The leaven went throughout the Corinthian church. In verse 7, Paul says, in essence, "You are supposed to be pure. Get that sin out, so you can repent." In verse 8, Paul defines leaven as "malice and wickedness." In other words, it is sin.
In Galatians 5:7-9, Paul calls leaven a "persuasion [that] does not come from Him who calls you," one that hinders us from obeying the truth. Putting these three verses together, this is how he defines leaven, as "a persuasion that does not come from God." In Luke 12:1, Jesus says that the leaven of the Pharisees is hypocrisy—hypocrisy in religion. In Mark 8:15, He speaks of "the leaven of the Pharisees and the leaven of Herod." Herod had leaven, too, and his was basically secularism or the use of religion for political purposes. Then, in Matthew 16:5-6, 11, Jesus clearly says that the leaven He spoke about was the doctrine of the Pharisees and the Sadducees.
So, then, what is leaven? In its most basic sense, it is a symbol of corruption, which has a tendency to multiply and spread like yeast. A little bit of yeast in the dough will make the whole thing rise because the yeast ferments and spreads throughout the entire lump of dough, making it all rise. In this parable leaven symbolizes sin that corrupts and spreads.