How is the kingdom of heaven like unto leaven?

Question

And what if a little leaven leaveneth the whole lump?

Seek first the kingdom of God,
and his righteousness(grace)?

Are we talking Law vs Grace?
Perhaps as Leaven vs No Leaven?

A little cancer us law?
Won't it kill the whole body, including the head?

Answer 1

The leaven is usually false doctrine, one false doctrine being attempted justification by the law which you properly identified.

Answer 2

Jesus understood that the Kingdom was already spread out on the earth. Most people don't see it, however. And a major reason they don't is because of their mistaken expectations about what the Kingdom is.

Jesus frequently told parables with the express design to upend such expectations. The classic two word example is "good Samaritan" -- for his Jewish audience, there was no such thing as a good Samaritan.

I suspect with his parable of the Kingdom being like leaven "hidden" in fifty pounds flour, Jesus was suggesting that the Kingdom is like a living, unexpected and active presence that suffuses everything we touch and see.

Here are comments from the Jesus seminar about the parable of the leaven:


The Kingdom of Heaven is like leaven which a woman took and concealed in fifty pounds of flour until it was all leavened. (Matthew 13:33)

This parable transmits the voice of Jesus as clearly as any ancient record can ....

In this one-sentence parable, Jesus employs three images in ways that would have been striking to his audience. The woman takes leaven and “conceals” it in flour. “Hiding” leaven in flour is an unusual way to express the idea of mixing yeast and flour. The surprise increases when Jesus notes that there were “fifty pounds” of flour. Three men appear to Abraham in Genesis 18 as representatives of God. They promise him and his wife, Sarah, that she will bear a son the following spring, although she was beyond the age of childbearing. For the occasion, Sarah is instructed to make cakes of fifty pounds of flour to give to the heavenly visitors. Fifty pounds of flour, it seems, is a suitable quantity to celebrate an epiphany, a visible, though indirect, manifestation of God. The third surprising figure in this one-line parable is the use of leaven.

Jesus employs the image of the leaven in a highly provocative way. In Passover observance, Judeans regarded leaven as a symbol of corruption, while the lack of leaven stood for what was holy. [During the Passover season, Judeans were under mandate to get all forms of yeast out of the house; the sacred bread had to be unleavened.] In a surprising reversal of the customary associations, the leaven here represents not what is corrupt and unholy, but God’s imperial rule .... (The Five Gospels, p. 195)
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Answer 3

The point being:

Don't let false doctrine creep into the church.
We need to use discernment wisely in the faith.