A mustard seed is very small, and yet, when it grows it turns into a large plant resembling a small tree. The Lord uses this imagery, which would have been very familiar to his listeners, to get across a very important point: God, and all that comes from God in our faith, enters discreetly into our lives; using other people, He sows seeds in our hearts; He waters them through other people still, and He waits patiently for them to grow. In this way, the Kingdom of God spreads, almost without being noticed, throughout the world.
The kingdom is also like the leaven that a woman puts into dough, and which makes it rise. So, the presence of God in the world is often times very discreet and hidden, and yet it is what gives the world to be, even in an age of darkness and violence, a place in which beauty and goodness can be seen. Someone like St Teresa of Calcutta gives us a picturesque example of this. Starting out alone, unknown and weak, she began to visit the poorest of the poor and look after them. Little by little, her influence, like that of yeast in the dough, brought forth an abundant harvest of heroic souls who put themselves at the service of others, being living models of God's love in the world.
We can all identify moments in our past when we received a seed, or when a bit of yeast was hidden in our heart: it may have been a pious thought, an encouragement, a good example or a lesson on a fundamental point of our faith. It may have come to us through a parent, a relative, a teacher, a priest, a nun, perhaps even through a total stranger. At the time it may have seemed small, perhaps even insignificant, but it took root in us and grew into a big tree which produces fruit today.
Let us be ever conscious of these very discreet workings of the Holy Spirit in our souls. Let us keep the ears of our heart open to perceive the inspirations we might receive and that the Lord wants us to share with others. And let us take to heart, with humility, the words of others that are destined to help and instruct us. In this way, the kingdom of God will grow in our minds and hearts, and will spread in the souls of good faith who are out there, like fields waiting for the good seed.
Finally, let us give thanks for those who sowed the good seed in our lives. In eternity we shall see how much we owe to them of what we are and were able to accomplish. It's one of the glories of the "communio sanctorum": we are never alone, but must help each other save our souls and give glory to the Triune God for all eternity.