Leaven — a Small Thing That Affects Everything
The image of leaven in the Scripture of the New Testament is used not as something bad or good in itself, but to show how a small part can affect all the rest. Little leaven is put into the flour, but in the end it leavens the whole lump.
Besides leaven, the Lord also often turns to the image of salt, which likewise affects everything else (preserves it from corruption):
“You are the salt of the earth; but if the salt loses its flavor, how shall it be seasoned? It is then good for nothing but to be thrown out and trampled underfoot by men” (Matt. 5:13)
“Salt is good, but if the salt loses its flavor, how will you season it? Have salt in yourselves, and have peace with one another” (Mark. 9:50)
At the same time salt, like leaven, took part in the Old Testament sacrifices:
“And every offering of your grain offering you shall season with salt; you shall not allow the salt of the covenant of your God to be lacking from your grain offering. With all your offerings you shall offer salt” (Lev. 2:13)
Positive Meanings of Leaven
Leaven — the Kingdom of Heaven
The Lord Jesus Christ compares leaven with the Kingdom of Heaven. Again, something small is to affect everything:
“Another parable He spoke to them: ‘The kingdom of heaven is like leaven, which a woman took and hid in three measures of meal till it was all leavened’” (Matt. 13:33)
“And again He said, ‘To what shall I liken the kingdom of God? It is like leaven, which a woman took and hid in three measures of meal till it was all leavened’” (Luke. 13:20-21)
Negative Meanings of Leaven
Leaven — Heretical Teaching
The Lord Jesus Christ points out to the apostles the perniciousness of the teaching of the Pharisees and Sadducees. He uses the image of leaven to show how something small can affect everything:
“Then Jesus said to them, ‘Take heed and beware of the leaven of the Pharisees and the Sadducees.’ <...> ‘How is it you do not understand that I did not speak to you concerning bread? — but to beware of the leaven of the Pharisees and Sadducees.’ Then they understood that He did not tell them to beware of the leaven of bread, but of the doctrine of the Pharisees and Sadducees” (Matt. 16:6,11-12)
Leaven — Hypocrisy
On another occasion, before thousands of people, Christ again uses the image of leaven to point out the perniciousness of the distinguishing trait of the Pharisees — hypocrisy. Again, hypocrisy turns out to be something small that affects everything:
“In the meantime, when an innumerable multitude of people had gathered together, so that they trampled one another, He began to say to His disciples first of all, ‘Beware of the leaven of the Pharisees, which is hypocrisy’” (Luke. 12:1)
Leaven — an Unrepentant Fornicator in the Community
The apostle Paul uses the image of leaven to reproach the Corinthian community for having an unrepentant fornicator among them. The fornicator is a small part of the community, but can influence the whole community:
“It is actually reported that there is sexual immorality among you, and such sexual immorality as is not even named among the Gentiles — that a man has his father’s wife! <...> Your glorying is not good. Do you not know that a little leaven leavens the whole lump? Therefore purge out the old leaven, that you may be a new lump, since you truly are unleavened. For indeed Christ, our Passover, was sacrificed for us. Therefore let us keep the feast, not with old leaven, nor with the leaven of malice and wickedness, but with the unleavened bread of sincerity and truth” (1 Cor. 5:1,6-8)
The apostle proposes getting rid not of leaven in general, but of the leaven of malice and wickedness: that is, expelling the unrepentant fornicator from the community (1 Cor. 5:9,11). Here the apostle reveals the Old Testament meanings, and is not speaking about bread:
“Therefore purge out the old leaven, that you may be a new lump, since you truly are unleavened. For indeed Christ, our Passover, was sacrificed for us. Therefore let us keep the feast, not with old leaven, nor with the leaven of malice and wickedness, but with the unleavened bread of sincerity and truth” (1 Cor. 5:7-8)
“How is it you do not understand that I did not speak to you concerning bread?” (Matt. 16:11)
Leaven — a Person Drawing the Community into Judaism
The apostle Paul uses the image of leaven, warning the Galatian community of the danger of having among them certain people who teach that one must keep the Law of Moses in order to be saved:
“You have become estranged from Christ, you who attempt to be justified by law; you have fallen from grace. For we through the Spirit eagerly wait for the hope of righteousness by faith. <...> A little leaven leavens the whole lump. I have confidence in you, in the Lord, that you will have no other mind; but he who troubles you shall bear his judgment, whoever he is. <...> I could wish that those who trouble you would even cut themselves off!” (Gal. 5:4-5,9-10,12)