What is the parable of the mustard seed?
The parable of the mustard seed was spoken by Jesus Christ and is recorded in Matthew 13:31-32 (also Mark 4:31-32; Luke 13:18-19). If in the tares we perceive a stage of Christ's teaching more advanced than in the sower, we may take the mustard seed as announcing the outward manifest triumph of His Kingdom, while the leaven discloses to us the secret of its inward working. Strange difficulties have been started by Westerners who had never set eyes on the luxuriant growth of the mustard plant in its native home, and who demur to the letter which calls it "the least of all seeds."
But in the Koran (Sura xxxi) this proverbial estimate is implied; and it is an elementary rule of sound Scripture criticism not to look for scientific precision in such popular examples, or in discourses which aim at something more important than mere knowledge. The tree, salvadora persica, is said to be rare. Obviously, the point of comparison is directed to the humble beginnings and extraordinary development of Christ's Kingdom. Wellhausen believes that for the Evangelists the parable was an allegory typifying the Church's rapid growth; Loisy would infer that, if so, it was not delivered by our Lord in its actual form.
31 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:32 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.
Here are three distinct yet cognate stories, the mustard seed, the leaven, the seed growing secretly, occurring in the Synoptics, contemplating a lapse of time, and more applicable to after-ages than to the brief period during which Christ was preaching; shall we say that He uttered none of them? And if we allow these prophetic anticipations at all, does not the traditional view explain them best? (Wellh., "Matt.", 70; Loisy, "Ev. syn.", III, 774-3.)
It has been questioned whether in the leaven we should recognize a good influence, answering to the texts, "you are the salt of the earth, the light of the world" (Matthew 5:13-14), or the evil to be "purged out" according to St. Paul (I Corinthians 5:6-8). Better to take it as the "good seed", with consequent applications, as St. Ignatius does (Ad Magnes., x) and St. Gregory Nazianzus (Orat., xxxvi, 90).
By the "three measures" were understood in the Gnostic system the "earthly" "carnal", and "spiritual" classes among Christians (Iren., I, viii). Trench admirably describes these two parables as setting before us the "mystery of regeneration" in the world and the heart of man. For the "leaven of the Pharisees", consult authors on Matthew 16:6.
International Standard Bible Encyclopedia, which is in the public domain (with minor edits).