The Kingdom of God, like a mustard seed, grows significantly from humble beginnings.
Luke 13:18-19 RSV - He said therefore, "What is the kingdom of God like? And to what shall I compare it? It is like a grain of mustard seed which a man took and sowed in his garden; and it grew and became a tree, and the birds of the air made nests in its branches."
Messiah taught about the Kingdom of God using many different parables or brief, symbolic representations. In this instance, he compares the Kingdom of God to the life cycle of a mustard seed. It is one of the smallest seeds that exist, yet mustard seeds can grow to be large enough for people to sit under for shade or for birds to come and make nests within its branches.
Parables were designed to share spiritual truths but in symbolic ways. Even in the gospel narratives, not everyone understood all of the parables, but many times in private, Messiah would reveal the meaning of the symbolism to his disciples.
I believe the imagery Messiah uses here is drawn from the prophets, specifically Daniel. In one instance, Daniel was asked to interpret the dream of Nebuchadnezzar, who, at that time, was the dominant leader of the known world. King Nebuchadnezzar had dreamt of a large tree and shared the details with Daniel. Daniel replied:
Daniel 4:20-22 RSV - The tree you saw, which grew and became strong, so that its top reached to heaven, and it was visible to the end of the whole earth; whose leaves were fair and its fruit abundant, and in which was food for all; under which beasts of the field found shade, and in whose branches the birds of the air dwelt-- it is you, O king, who have grown and become strong. Your greatness has grown and reaches to heaven, and your dominion to the ends of the earth.
This great tree, i.e., Nebuchadnezzar's kingdom, had grown to have dominion over the known world. Messiah uses almost the exact same imagery in his parable of the Kingdom of God: "it grew and became a tree," and "the birds of the air made nests in its branches."
One of the early church fathers (that is, one of the early Christian leaders and thinkers) had this to say regarding this parable:
"The seed of the Gospel is the least of seeds, because the disciples were weaker than the whole of mankind; yet forasmuch as there was great might in them, their preaching spread throughout the whole world..." -John Chrysostom (347-407 AD).
The original parable, while having many different interpretations assigned to various parts of the word picture, essentially explains itself when simply taken at face value. We know it is about the Kingdom of God because that is the comparison being made. It was explained as beginning as a very small seed. Consider that the movement began with Messiah and his twelve disciples. Yet, through their teaching and traveling in response to the Great Commission, they were able to spread the teaching of the Kingdom throughout the known world in less than forty years under the bitterest of persecution. As a result, the "greatness [of God's Kingdom] has grown and reaches to heaven, and [God's] dominion to the ends of the earth."
Matthew 28:18-20 RSV - And Jesus came and said to them, "All authority in heaven and on earth has been given to me. Go therefore and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit, teaching them to observe all that I have commanded you; and lo, I am with you always, to the close of the age."
Due to their faithfulness, the parable of the mustard seed became a reality in their generation. It is now up to each successive generation, with the abilities provided by God, to continue to work to reach the farthest limits of their own generations with the truths of the Kingdom of God.