John Chrysostom

And also because the wisdom spoken amongst the perfect expands, to an extent greater than all other sayings, that which was told to men in short discourses, for there is nothing greater than this truth.

Catena Aurea by Aquinas ↗

Theophylact of Ohrid

The word of faith is small: because one need only believe in Christ, and you shall be saved. You see that this word is as small as a mustard seed.

Commentary on Mark ↗

Clement of Alexandria

The word which proclaims the kingdom of heaven is sharp and pungent as mustard. It represses bile (anger) and checks inflammation (pride). From this word flows the soul's true vitality and fitness for eternity. To such increased size did the growth of the word come that the tree which sprang from it (that is the Church of Christ now being established over the whole earth) filled the world, so that the birds of the air (that is, holy angels and lofty souls) dwelt in its branches.

Nicetas Bishop of Heraclea ↗

Bede

When it is sown in the ground, he says, it is smaller than all the seeds that are on the earth. But when it is sown, it grows up and becomes greater than all garden plants and puts forth large branches, so that the birds of the air can make nests under its shade. The preaching of the Gospel is the smallest among all disciplines. For the first teaching indeed does not hold the truth of faith, proclaiming a man, Lord God, dead and the scandal of the cross. …

On the Gospel of Mark ↗

Hippolytus of Rome

The Phrygians, however, further assert that the father of the universe is "Amygdalus," not a tree, he says, but that he is "Amygdalus" who previously existed; and he having in himself the perfect fruit, as it were, throbbing and moving in the depth, rent his breasts, and produced his now invisible, and nameless, and ineffable child. respecting whom we shall speak. …

Hippolytus Refutation of All Heresies Book V ↗