Leaven has a negative meaning in Scripture and is a representation of sin, especially false doctrine. A little bit of it can have great consequences. A little leaven leavens the whole lump.
33 Another parable he spoke to them: The kingdom of heaven is like leaven, which a woman takes and hides in three measures of meal, until the whole is leavened.
flour
Flour is made into bread and it has a positive meaning. It is made from wheat and so we find a parallel with another parable in this passage, the parable of the tares, where the wheat seed is a representation of the Word of the Son of Man and the wheat represents ‘the sons of the kingdom’ (:37-38).
Elijah
The widow of Zarephath is in danger of dying of hunger with her son and has an encounter with Elijah, the man of God. He speaks to her and his word gives life. After that, she has no lack of oil and flour. In this story, flour is a representation of the living Word of God (1 Kings 17:9-16).
Elisha
We also find a story in which flour plays a role in Elisha. There, the sons of the prophets want to prepare food and one of the men puts the wrong fruit in the pot of pottage. It cannot be eaten and they cry out: “Man of God, death is in the pot!” (2 Kings 4:40). Elisha then had flour brought and threw it in the pot, after which it could be eaten. Here too, flour is a representation of the Word of God, which gives life.
three measures of flour
Once again, elsewhere in the Bible, there is mention of three measures of flour. When Abraham receives a visit from YAHWEH in Genesis 18, represented by three men, he has his wife Sarah prepare cakes of bread from three measures of flour (Gen. 18:6). Three measures of flour is that which is intended for YAHWEH.
a woman hides
A woman hides leaven in that which is intended for the Lord. In the Bible, the woman is a picture of Israel or of creation. The woman represents the external things. That can be positive, because the woman is the glory of the man (2 Cor. 11:7).
secretly
In this parable it is negative. The woman is a representation of the world, which brings leaven into what is intended for the Lord. And that happens in a sneaky way. The woman hides the leaven in the three measures of flour.
Already in the letters of the New Testament we read how Paul warns about what at first glance may seem like minor adjustments that were made to his teaching. But he says that a little leaven leavens the whole lump.