We now come to a very short parable. Although there are a few more verses after it, the parable of the leaven is only described in one verse. As mentioned, the parable of the weeds, the mustard seed and the leaven form a series of parables that have a negative meaning.
33 Another parable he spoke to them: The kingdom of heaven is like leaven…
leaven
Leaven is old dough that has spoiled (>leavened) and that is used to make bread rise. Some commentators say that yeast is dough from the previous day, from. The old that is brought into the new. Leaven has a very negative meaning in the Bible.
unleavened
Israel ate unleavened bread at the Exodus from Egypt (Ex. 12:15) and this institution of the Passover was from that moment on one of the feasts of YAHWEH, which Israel would keep (Ex. 12:17; Lev. 23:6). Almost all the meal offerings were unleavened (Ex. 29:2; Lev. 6:16). That which was set apart for the Lord would contain no leaven.
sin
In 1 Corinthians 5, Paul points out to the Corinthians that he had heard that there was fornication among them, which he had never heard of even among the Gentiles (:1). Paul says that this could only exist because the Corinthians were puffed up (1 Cor.5:2), like leaven. Because they did not stand in the truth, such a person endured among them. He indicates that a little leaven leavens the whole lump and that this old leaven should be purged out (:6-7).
justified by works
Among the Galatians there were false teachers (2:4) who taught that if a believer wanted to live righteously, one had to be circumcised (:2). They had fallen from grace (5:4) and Paul also says of this: a little leaven leavens the whole lump (5:9). Leaven is a picture of false doctrine.
We also see this in:
Matthew 16
6 And Jesus said unto them, Take heed and consider the leaven of the Pharisees and Sadducees
The disciples consulted with each other about why Jesus said this. Was it because they had not brought any bread? Jesus then reminds them of the feeding of the four thousand and the feeding of the five thousand (:7-10). In other words: if He wants there to be bread, He will provide it Himself. The Lord is concerned with something else, with the typological meaning of leaven.
Matthew 16
11 How do you not understand that I did not say to you about the bread? Consider the leaven of the Pharisees and Sadducees.
12 Then they will understand that He did not say to consider the leaven of the bread, but the teaching of the Pharisees and Sadducees.
false doctrine
Leaven is a picture of the teaching of the Pharisees, Sadducees and also of the Herodians (Mark 8:15). We know from the Pharisees that they had placed traditions of men above the word of God, that the Sadducees did not believe in the resurrection and that the Herodians wanted to establish the Kingdom through political means. All contrary to sound doctrine and therefore leaven.