“Which indeed is the least of all seeds: but when it is grown, it is the greatest among herbs, and becometh a tree, so that the birds of the air come and lodge in the branches thereof.”
Matthew 13:32 — Greek Interlinear
Church Fathers on Matthew 13:32
Seeing the Lord had said above that three parts of the seed perish, and one only is preserved, and of that one part there is much loss by reason of the tares that are sown upon it; that none might say, Who then and how many shall they be that believe; He removes this cause of fear by the parable of the mustard seed.
The kingdom of heaven is the preaching of the Gospel, and the knowledge of the Scriptures which leads to life, concerning which it is said to the Jews, 'The kingdom of God shall be taken from you.' [Matt 21:43] It is the kingdom of heaven thus understood which is likened to a grain of mustard seed.
A grain of mustard seed may allude to the warmth of faith, or to its property as antidote to poison.