From: Luke 13:18-21
Parables of the Grain of Mustard Seed and of the Leaven
 He (Jesus) 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."
 And again He said, "To what shall I compare the Kingdom of God?  It is like leaven which a woman took and hid in three measures of meal, till it was all leavened."
18-21. The grain of mustard and the leaven symbolize the Church, which starts off as a little group of disciples and steadily spreads with the aid of the Holy Spirit until it reaches the ends of the earth. As early as the second century Tertullian claimed: "We are but of yesterday and yet we are everywhere" ("Apologeticum", 37).
Our Lord "with the parable of the mustard seed encourages them to have faith and shows them that the Gospel preaching will spread in spite of everything. The Lord's disciples were the weakest of men, but nevertheless, because of the great power that was in them, the Gospel has been spread to every part of the world" (St. John Chrysostom, "Hom. on St. Matthew", 46). Therefore, a Christian should not be discouraged if his apostolic action seems very limited and insignificant. With God's grace and his own faithfulness it will keep growing like the mustard seed, in spite of difficulties: "In the moments of struggle and opposition, when perhaps `the good' fill your way with obstacles, lift up your apostolic heart: listen to Jesus as He speaks of the grain of mustard seed and of the leaven. And say to Him: `"edissere nobis parabolam": explain the parable to me.' And you will feel the joy of contemplating the victory to come: the birds of the air lodging in the branches of your apostolate, now only in its beginnings, and the whole of the meal leavened" ([St] J. Escriva, "The Way", 695).
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.