Leaven is a substance that causes bread to ferment and rise.
Leaven was not allowed to be used with the Old Testament sacrifices. There was a period of time each year when leaven could not even be present in the houses of the Jews. Those who violated this commandment were to be "cut off from the congregation of Israel". Leaven is representative of some things that God hates and that he strongly warns us against. God abhors a mixing of sinful things with holy things.
“Seven days shall there be no leaven found in your houses: for whosoever eateth that which is leavened, even that soul shall be cut off from the congregation of Israel, whether he be a stranger, or born in the land.”
“Thou shalt not offer the blood of my sacrifice with leaven; neither shall the sacrifice of the feast of the passover be left unto the morning.”
In the New Testament leaven is representative of false doctrine, hypocrisy, malice, and wickedness. It is something to beware of and to purge out.
“And when his disciples were come to the other side, they had forgotten to take bread. Then Jesus said unto them, Take heed and beware of the leaven of the Pharisees and of the Sadducees. And they reasoned among themselves, saying, It is because we have taken no bread. Which when Jesus perceived, he said unto them, O ye of little faith, why reason ye among yourselves, because ye have brought no bread? Do ye not yet understand, neither remember the five loaves of the five thousand, and how many baskets ye took up? Neither the seven loaves of the four thousand, and how many baskets ye took up? How is it that ye do not understand that I spake it not to you concerning bread, that ye should beware of the leaven of the Pharisees and of the Sadducees? Then understood they how that he bade them not beware of the leaven of bread, but of the doctrine of the Pharisees and of the Sadducees.”
(Matthew 16:5- 12)
“And he charged them, saying, Take heed, beware of the leaven of the Pharisees, and of the leaven of Herod.”
“In the mean time, when there were gathered together an innumerable multitude of people, insomuch that they trode one upon another, he began to say unto his disciples first of all, Beware ye of the leaven of the Pharisees, which is hypocrisy.”
Why does God use leaven as an example of things to be strictly avoided (or completely eliminated once it is found to be present)? Because all that it takes is a little leaven to make a big change. We live in a foolish age. Much false teaching and other forms of sin have made their way into religious organizations and even into the hearts of true Christians because of the reasoning that "after all it is just a little thing...what harm could it do?".
All of Protestantism is leavened. This is evident from the Roman Catholic teaching that is in Protestant churches (infant baptism, "sprinkling baptism", celebration of Roman Catholic holy days, hierarchies within the church system, and much more). It is also seen in the history of Protestant persecution (torture, banishment, theft, murder of Bible believing Christians in Europe and in Colonial America). This only ended when civil government forced Protestantism to cease from this wickedness.
The leaven of worldly music has deceived so many today. It is a Trojan horse of false doctrine. The modern Bibles that contain so much corruption and false doctrine have fooled the unlearned and learned alike. Virtually all of the churches in America are leavened.
“Ye did run well; who did hinder you that ye should not obey the truth? This persuasion cometh not of him that calleth you. A little leaven leaveneth the whole lump.”
The best solution for the leaven of sin is preventative (do what Jesus says and simply avoid this "leaven" altogether). When this "leaven" has infiltrated a church or an individual’s heart then God commands that it be completely purged out.
“Your glorying is not good. Know ye not that a little leaven leaveneth the whole lump? Purge out therefore the old leaven, that ye may be a new lump, as ye are unleavened. For even Christ our passover is sacrificed for us: 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.”
(I Corinthians 5:6-8)