What does Luke 13:19 mean?
Jesus is describing the kingdom of God as something which begins exceedingly small. Then it grows, slowly but steadily, until completion. Once growth starts, it cannot be removed. Here, it is a small mustard seed that becomes a tree. Next, it is a little bit of leaven gradually worked through a great amount of flour (Luke 13:20–21).In the chapter before, Jesus touched on the same theme. A cloud, far off, becomes a thunderstorm. The southern wind becomes a scorching heat (Luke 12:54–56). The Jews know this theme well, as their nation started with a single grandson of Abraham. Even so, they are impatient and want the kingdom to come immediately (John 6:15).
In other passages, Jesus is recorded describing mustard as the "smallest" of seeds. His point is exaggeration for effect, not a claim that the mustard seed is the smallest in the entire plant kingdom. Rather, it is the smallest planted by Jewish gardeners. Further, mustard seeds were a metaphor for a ridiculously small quantity or size, often contrasted to massive plants such as cedar trees.
In the Old Testament, birds roosting in a tree represent a powerful nation or leader where people of many nations find shelter. For instance, Assyria (Ezekiel 31:6) and Babylon (Daniel 4:20–22). Although the parallel isn't obvious here, Jesus will soon describe the influx of Gentiles into God's kingdom: "And people will come from east and west, and from north and south, and recline at table in the kingdom of God" (Luke 13:29).