Grace and Space

Lk 13:18-21
Mustard seed or yeast: The smallness of the kingdom of God as revealed through the teaching, healing, and preaching ministry of Jesus in Luke is compared to a mustard seed, a very tiny seed.
The reference to the mustard seed growing to become large enough for the birds to dwell in its branches is a reference to and parody on the noble cedar, which represents Israel’s restoration, in Ezekiel (cf 17:22-23).
 Luke is declaring that Israel will be restored, but the metaphor for the kingdom is no longer greatness (the cedar) but smallness (the mustard seed). The surprising parallel that is drawn between the kingdom of God and the mustard seed is a departure from the usual description of its vastness.
Yeast, or leaven, was considered to be corrupt (evil) in the ancient world. Therefore, unleavened (non-corrupted) bread was prescribed for Jewish holy days, especially Passover. One could not use leaven in the presence of God.
In the parable, Jesus compares the kingdom of God to leaven! In other words, the kingdom of God corrupts everything it touches, just as leaven makes the whole batch of dough rise.
The agent of the kingdom is not a man but a woman. The ancient world had no regard for women, so to declare the agent of the kingdom to be a woman is to reduce its importance. This detail would have been highly regarded by Luke, since he portrays women as having a significant role in the ministry of Jesus and in the early Church.
By using this parable, Luke is declaring that the kingdom of God is appearing now through the most unlikely of people (women, sinners, tax collectors), and it is corrupting everyone with whom it makes contact.