From: Matthew 13:31-35
The Mustard Seed; The Leaven
 Another parable He (Jesus) put before them saying, "The Kingdom of Heaven is like a grain of mustard seed which a man took and sowed in his field;  it is the smallest of all seeds, but when it has grown it is the greatest of shrubs and becomes a tree, so that the birds of the air come and make nests in its branches."
 He told them another parable. "The Kingdom of Heaven is like a leaven which a woman took and hid in three measures of meal, till it was all leavened."
 All this Jesus said to the crowds in parables; indeed He said nothing to them without a parable.  This was to fulfill what was spoken by the prophet: "I will open my mouth in parables, I will utter what has been hidden since the foundation of the world."
31-32. Here, the man is Jesus Christ and the field, the world. The grain of mustard seed is the preaching of the Gospel and the Church, which from very small beginnings will spread throughout the world.
The parable clearly refers to the universal scope and spread of the Kingdom of God: the Church, which embraces all mankind of every kind and condition, in every latitude and in all ages, is forever developing in spite of obstacles, thanks to God's promise and aid.
33. This comparison is taken from everyday experience: just as leaven gradually ferments all the dough, so the Church spreads to convert all nations.
The leaven is also a symbol of the individual Christian. Living in the middle of the world and retaining his Christian quality, he wins souls for Christ by his word and example: "Our calling to be children of God, in the midst of the world, requires us not only to seek our own personal holiness, but also to go out onto all the ways of the earth, to convert them into roadways that will carry souls over all obstacles and lead them to the Lord. As we take part in all temporal activities as ordinary citizens, we are to become leaven acting on the mass" ([St] J. Escriva, "Christ Is Passing By", 120).
34-35. Revelation, God's plans, are hidden (cf. Matthew 11:25) from those who are disposed to accept them. The Evangelist wishes to emphasize the need for simplicity and for docility to the Gospel. By recalling Psalm 78:2, he tells us once more, under divine inspiration, that the Old Testament prophecies find their fulfillment in our Lord's preaching.
Source: "The Navarre Bible: Text and Commentaries". Biblical text taken from the Revised Standard Version and New Vulgate. Commentaries made by members of the Faculty of Theology of the University of Navarre, Spain. Published by Four Courts Press, Kill Lane, Blackrock, Co. Dublin, Ireland. Reprinted with permission from Four Courts Press and Scepter Publishers, the U.S. publisher.