Another parable put he forth unto them, saying, The kingdom
of heaven is like to a grain of mustard seed, which a man took, and
sowed in his field: 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. Another parable spake he unto them; The
kingdom of heaven is like unto leaven, which a woman took, and hid in
three measures of meal, till the whole was leavened. All
these things spake Jesus unto the multitude in parables; and without a
parable spake he not unto them: That it might be
fulfilled which was spoken by the prophet, saying, I will open my mouth
in parables; I will utter things which have been kept secret from the
foundation of the world. Then Jesus sent the multitude
away, and went into the house: and his disciples came unto him, saying,
Declare unto us the parable of the tares of the field.
I found something labeled "The Apostolic Fathers with Justin Martyr and Irenaeus" that, ironically, touches both on the mustard seed topic and the twelve apostles who are commemorated today…
"Observe that, by means of the grain of mustard seed in the parable, the heavenly doctrine is denoted which is sown like seed in the world, as in a field, [seed] which has an inherent force, fiery and powerful. For the Judge of the whole world is thus proclaimed, who, having been hidden in the heart of the earth in a tomb for three days, and having become a great tree, has stretched forth His branches to the ends of the earth. Sprouting out from Him, the twelve apostles, having become fair and fruitful boughs, were made a shelter for the nations as for the fowls of heaven, under which boughs, all having taken refuge, as birds flocking to a nest, have been made partakers of that wholesome and celestial food which is derived from them."
Gregory the Great in his Book V, Letter 43 of Registrum Epistolarum said:
"…we give thanks the more to that grain of mustard seed, for that from what appeared a small and despicable seed it has been so spread everywhere by branches rising and extending themselves from the same root that all the birds of heaven may make their nests in them…"
At the Second Ecumenical Council in Constantinople A.D.381, St. Gregory Nazianzen was the final orator. He gave a beautiful address (full text here of Oration 42, Read it!!!!). The audience was the 150 bishops of the Eastern church, as well as the Christians of Constantinople. St. Gregory had brought them together with much toil as they had been "ravaged by heretical teachers" and "in a language of exquisite beauty, spoken with the quivering tones of an aged man, he bids a tender farewell to his flock, his cathedral, and his throne…" (New Advent.org)
"Go through My gates (Is.62:10) and be ye enlarged. Must you always be in trouble and dwell in tents, while those who vex you rejoice exceedingly? And to the presiding Angels, for I believe, as John teaches me in his Revelation, that each Church has its guardian (Rev. 2:1). Prepare ye the way of My people, and cast away the stones from the way (Is.62:10) that there may be no stumbling block or hindrance for the people in the divine road and entrance, now, to the temples made with hands, (Acts 7:48) but soon after, to Jerusalem above, (Gal 4:26) and the Holy of holies there, which will, I know, be the end of suffering and struggle to those who here barely travel on the way. Among whom are you also called to be Saints (Rom 1:6) a people of possession, a royal priesthood, (I Pet 2:9) the most excellent portion of the Lord, a whole river from a drop, a heavenly lamp from a spark, a tree from a grain of mustard seed, (Matt 13:21) on which the birds come and lodge."
(Might add more later, so much on this passage in early church writings…but have to get moving!)