Ambrose of Milan

Many think Christ is the leaven, for leaven which is made from meal, excels its kind in strength, not in appearance. So also Christ (according to the Fathers) shone forth above others equal in body, but unapproachable in excellence. The Holy Church therefore represents the type of the woman, of whom it is added, Which a woman, took and hid in three measures (sata) of meal, till the whole was leavened. …

Catena Aurea by Aquinas ↗

Bede

And again he said, "To what shall I compare the kingdom of God, and what is it like? It is like leaven that a woman took and hid in three measures of flour until the whole was leavened." A measure called "satum" is a unit of measure according to the custom of the province of Palestine, holding one and a half modii. Therefore, the leaven refers to love, which heats up and stirs the mind. …

On the Gospel of Luke ↗

Bonaventure

And again he said: To what is it like, etc. After he set forth an example inciting to continuous progress in the knowledge of truth, here he adds an example inciting to progress in the fervor of charity, and this under the metaphor of leaven, from whose likeness he incites to continuous progress in charity with respect to its threefold analogous property, namely that which leaven has on the part of its own nature, on the part of another's industry, and on the part of both. …

Commentary on Luke, Chapter 13 ↗