‘He set another parable before them, saying, “The kingly rule of heaven is like to a situation where a man took a grain of mustard seed, and sowed it in his field, which indeed is less than all seeds, but when it is grown, it is greater than the herbs, and becomes a tree, so that the birds of the heaven come and lodge in its branches.” '
The emphasis in this parable is on organic growth from small beginnings. Whether in Jesus' mind the ‘grain of mustard seed' is the kingly Rule itself, or the small band of disciples, does not really matter. The point is that what starts out as something very small will become something very substantial. A grain of mustard seed was very small, the smallest known in Palestine. (‘Less than all the seeds' has in mind the seeds with which a Palestinian farmer would be familiar. To the Rabbis the mustard seed was proverbial for its smallness. Or it may indicate that the farmer selected the smallest of all the mustard seeds for planting). This emphasises the tiny beginnings of the Kingly Rule of Heaven (‘fear not, little flock, it is your Father's good pleasure to give you the Kingly Rule' - Luke 12:32). The field is clearly the world, in which the Kingly Rule of Heaven is planted. And the Kingly Rule then grows into a ‘tree' (very large bush). The mustard was in fact the only herb that grew to such a great size in contrast with the size of the seed. Mustard trees/bushes can often grow to over two metres (seven feet) tall, and even more. But as this demonstrates, had Jesus had intended simply to indicate hugeness He would actually have chosen a tree. The emphasis here is clearly rather on the growth from small beginnings.
The fact that the birds of heaven came and lodged in its branches accentuates its size, but they may well also be intended to indicate the nations of the world because that is precisely what they indicated in an Old Testament parable (Ezekiel 31:6; see also Ezekiel 17:22-24; Ezekiel 31:3-14; Daniel 4:7-23). Thus the Kingly Rule of Heaven will grow from tiny beginnings to something so surprisingly substantial (even though it is a herb) that the nations of the world will be able to find shelter in it (Matthew 12:18; Matthew 12:21). It is a phenomenon.