What does Luke 13:21 mean?
Jesus is telling two short parables about the inevitability of the coming kingdom of God. First, He compared it to a mustard seed: the smallest seed sown in a Jewish garden. The seed is planted and a tough tree slowly grows, providing shade and a place to roost for birds. Birds nested in trees often refer to people of many nations finding rest under the protection of a powerful kingdom and/or leader (Ezekiel 31:6; Daniel 4:20–22). Jesus' presence inaugurated the kingdom of God, and it will inevitably grow and welcome people of all nations (Luke 13:29).Next, Jesus compares the kingdom to leavening stirred into an enormous batch of flour. Leavening is anything that causes a chemical reaction that makes dough rise. This can include baking soda, baking powder, or yeast. Once it is added to the flour, the flour and leaven cannot be separated again. As the woman continues to stir, the leavening spreads throughout the flour until every bit is affected.
Although leaven is often a metaphor for something negative (Luke 12:1; 1 Corinthians 5:6; Galatians 5:7–9), here it is positive. The kingdom of God will permeate the entire world. Just as a little leaven works its way through the entire batch, once the kingdom of God is introduced its eventual completion is inevitable. The very nature of the world has been changed with Jesus' arrival and will continue to be through the ministry of the Holy Spirit.
Some Bible scholars translate the "three" measures as a metaphor to represent the fullness of the human experience: body/soul/spirit, earth/church/state, Jews/Samaritans/Greeks. There's no indication this is what Jesus means. He's simply referring to a huge quantity of flour to represent the large world.