Theophylact of Ohrid — as excerpted in the Catena Aurea on Luke 13:18-21

“Or, any man receiving a grain of mustard seed, that is, the word of the Gospel, and sowing it in the garden of his soul, makes it a great tree, so as to bring forth branches, and the birds of the air (that is, they who soar above the earth) rest in the branches, (that is, in sublime contemplation.) For Paul received the instruction of Ananias (Acts 9:17.) as it were a small grain, but planting it in his garden, he brought forth many good doctrines, in which they dwell who have high heavenly thoughts, as Dionysius, Hierotheus, and many others. He next likens the kingdom of God to leaven, for it follows, And again he says, Whereunto shall I liken it? It is like to leaven, &c.”