Luke 13:18

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. (Luke 17:21.) 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 odour 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 savour of Christ. (2 Cor. 2:15.) 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 a far off now fly, having taken up the wings of the virtues. Sow then Christ in thy garden; a garden is truly a place full of flowers, wherein the grace of thy work may blossom, and the manifold odour of thy different virtues be breathed forth. Wherever is the fruit of the seed, there is Christ.

Or else; The kingdom of God is the Gospel, through which we gain the power of reigning with Christ. As then the mustard seed is surpassed in size by the seeds of other herbs, yet so increases as to become the shelter of many birds; so also the life-giving doctrine was at first in the possession only of a few, but afterwards spread itself abroad.

While His adversaries were ashamed, and the people rejoiced, at the glorious things that were done by Christ, He proceeds to explain the progress of the Gospel under certain similitudes, as it follows, Then said he, Unto what is the kingdom of God like? It is like a grain of mustard seed, &c. (Mat. 17:19.)