Matthew 13:31-32 (New International Version)
31 He told them another parable: “The kingdom of heaven is like a mustard seed, which a man took and planted in his field. 32 Though it is the smallest of all seeds, yet when it grows, it is the largest of garden plants and becomes a tree, so that the birds come and perch in its branches.”
Jesus spoke in many parables, instructing people in terms of what they could understand. He often used familiar circumstances and objects from their daily lives, so that those who were meant to understand his teaching could do so. On the surface these parables seem to be just simple stories, but when if you meditate on them and seek God's wisdom in them, they take on a much bigger meaning.
The parable of the mustard seed is one such parable. Jesus used the tiniest of seeds in his time to illustrate many concepts in his teaching. In my opinion, Jesus uses the mustard seed to illustrate what was to happen in the Kingdom of Heaven that Jesus brought with him. We understand that the kingdom he is talking about is the reign of Christ in the hearts of mankind, and the belief and obedience we have in faith in him.
The Kingdom of Heaven started in the smallest of towns in a remote corner of the world, as a baby in the manager. God planted his Son in the world to live among us and proclaim the message of the Kingdom of Heaven. His message of the gospel of Jesus Christ and his sacrifice on our behalf took root, grew and spread throughout the world, and those who took hold and believed the message were able to find rest and comfort in the shelter of his love, mercy and grace.