Theophylact of Ohrid
Catena Aurea · Lukasevangelium 13
Then said he, Unto what is the kingdom of God like? and whereunto shall I resemble it? It is like a grain of mustard seed, which a man took, and cast into his garden; and it grew, and waxed a great tree; and the fowls of the air lodged in the branches of it. And again he said, Whereunto shall I liken the kingdom of God? It is like leaven, which a woman took and hid in three measures of meal, till the whole was leavened.
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.