When I was four years old, I had the most beautiful front yard in the world. It was a bright green canopy covered in little yellow flowers. I would pick the yellow flowers as bouquets for my mom. I thought it was magical.
My dad had a different perspective. He cursed the dandelions because he knew what I didn't... that they were a weed. Left unchecked, they would eventually take over the whole yard, killing the grass and anything else we might want to grow on its borders.
A mustard seed is a weed. Once planted, it will explode, growing wherever it wants, and will quickly become huge and unruly. For Jesus to compare God's word to a mustard seed is more than just a tiny thing that grows large. It's a seemingly inane seed that quickly explodes and becomes unable to be contained. To some, God's word might seem a weed that needs to be contained before it spreads. To others, God's word is a simple thing to plant, and then it takes on a life of its own - apart from my nurture, care, watering, or coaxing. God's word, once planted, explodes and quickly cannot be contained. To these eyes, God's word is a pretty little yellow flower, and the hostile takeover is a magical thing to behold.
He said therefore, “What is the kingdom of God like? And to what should I compare it? It is like a mustard seed that someone took and sowed in the garden; it grew and became a tree, and the birds of the air made nests in its branches.” And again he said, “To what should I compare the kingdom of God? It is like yeast that a woman took and mixed in with three measures of flour until all of it was leavened.” Jesus went through one town and village after another, teaching as he made his way to Jerusalem.
Someone asked him, “Lord, will only a few be saved?” He said to them, “Strive to enter through the narrow door; for many, I tell you, will try to enter and will not be able. When once the owner of the house has got up and shut the door, and you begin to stand outside and to knock at the door, saying, ‘Lord, open to us,’ then in reply he will say to you, ‘I do not know where you come from.’ Then you will begin to say, ‘We ate and drank with you, and you taught in our streets.’ But he will say, ‘I do not know where you come from; go away from me, all you evildoers!’ There will be weeping and gnashing of teeth when you see Abraham and Isaac and Jacob and all the prophets in the kingdom of God, and you yourselves thrown out. Then people will come from east and west, from north and south, and will eat in the kingdom of God. Indeed, some are last who will be first, and some are first who will be last.”