The parable of the grain of mustard seed is a mystery, as the Fathers have said, and we are called to imitate its example. It is written, “The kingdom of heaven is like a grain of mustard seed, which a person took and sowed in his field. It is the smallest of all seeds, but when it has grown it is the greatest of herbs and becomes a shrub, so that the birds of the sky come and make nests in it’s branches.” [Mt 13:31]
This, then, is the grain of mustard seed, and these are its virtues which we are called to imitate in every way. When it says, “it is the smallest of all seeds“, it is referring to humility, that we must be subjected to all people. Its growth signifies meekness and longsuffering. Its redness means purity, not having any stain in the flesh. Its sharp twigs are the hatred of the passions, for such hatred is bitter for those who still desire wordly things. Its sweetness, which is only activated when it is mashed or threshed, signifies endurance. Its thresher is stung in the eyes on account of its powerful affliction. It is used to pickle dead things in order that they do not stink. Let us understand this and do likewise, dipping in it the dead parts of our soul so that they are not exposed to stench or worms.
This is why the Lord Jesus became human, in order that we may be concerned with endeavouring to behave as he did, searching ourselves as best we can in accordance with his example, asking whether or not we are like that seed, its condition humility, its sweetness and bitterness and taste. His mercy will strengthen us according to his will, for his is the glory, of the Father, and the Son, and the Holy Spirit, to the ages of ages. Amen.
[Abba Isaiah of Scetis, Ascetical Discourses, Discourse 11]