Luke 13:18-21
18 Then he said, "What is the kingdom of God like? To what can I compare it?
19 It is like a mustard seed that a person took and planted in the garden. When it was fully grown, it became a large bush and 'the birds of the sky dwelt in its branches.'"
20 Again he said, "To what shall I compare the kingdom of God?
21 It is like yeast that a woman took and mixed (in) with three measures of wheat flour until the whole batch of dough was leavened."
-----------
I'll leave this one to St. Ambrose:
AMBROSE; In another place, a grain of mustard seed is introduced where it is compared to faith. If then the mustard seed is the kingdom of God, and faith is as the grain of mustard seed; faith is truly the kingdom of heaven, which is within us. A grain of mustard seed is indeed a mean and trifling thing, but as soon as it is crushed, it pours forth its power. And faith at first seems simple, but when it is buffeted by adversity, pours forth the grace of its virtue.
The martyrs are grains of mustard seed. They have about them the sweet odor of faith, but it is hidden. Persecution comes; they are smitten by the sword; and to the farthest boundaries of the whole world they have scattered the seeds of their martyrdom.
The Lord Himself also is a grain of mustard seed; He wished to be bruised that we might see that we are a sweet savor of Christ. He wishes to be sown as a grain of mustard seed, which when a man takes he puts it into his garden. For Christ was taken and buried in a garden, where also he rose again and became a tree, as it follows, And it waxed into a great tree. For our Lord is a grain when He is buried in the earth, a tree when He is lifted up into the heaven.
He is also a tree overshadowing the world, as it follows, And the fowls of the air rested in his branches; that is, the heavenly powers and they whoever (for their spiritual deeds) have been thought worthy to fly forth. Peter is a branch, Paul is a branch, into whose arms, by certain hidden ways of disputation, we who were afar off now fly, having taken up the wings of the virtues. Sow then Christ in your garden; a garden is truly a place full of flowers, wherein the grace of your work may blossom; and the manifold odor of your different virtues be breathed forth. Wherever is the fruit of the seed, there is Christ.
