What does Matthew 13:32 mean?
This parable began with an unexpected, even jarring comparison, from the viewpoint of ancient Israel. Christ has compared the kingdom of heaven to a tiny mustard seed (Matthew 13:31). Israelites would have known exactly what Jesus meant by the kingdom of heaven. They understood that it was promised in Scripture, and it would be glorious.Now Jesus finishes the comparison, explaining that glory is exactly how the process will end. That tiny mustard seed grows into the largest of all the garden plants. Though the tree sometimes resembles a bush, it can also grow as tall as 8–12 feet high. The tree grows large enough to host nests for birds.
Christ depicts the kingdom of God as starting very small, then growing to enormous size. In the context of garden plants of that era, this was exactly the role mustard played: a notably tiny beginning with a notably massive result. Spiritually, Jesus is teaching that the kingdom will not appear in full glory out of nothing. It will begin tiny, in the hearts of Jesus' disciples, and it will grow from there as more and more people come to faith in Jesus. Eventually, Jesus will return and establish His glorious kingdom on earth, populated by all those who swore allegiance to the King.
Critics have occasionally made much of Jesus referring to mustard as "the smallest of all seeds." Among all the plants on earth, and even in the middle east, there are those with even tinier seeds. Such complaints miss the purpose of the parable, and its audience. Jesus' listeners understood His meaning as much as a modern person would understand an expression such as "my brother is the biggest football player, but he's sensitive." Literalism is not the point—the contrast is what's being implied.
Further, literature of that era would often use mustard seeds as the go-to metaphor for smallness, much as modern English speakers often do with "a grain of sand."