These two parables follow those of the "Sower" and of the "Wheat and Tares". Like those, these are simple narrations of well-known commonplace facts known by the crowd that time. The disciples did not ask the Lord to explain these two, probably because they had no trouble understanding their spiritual application to the Kingdom of God.
However, for lack of an explanation given with authority by the Master, there are different interpretations nowadays, in part for not considering sufficiently what these facts represent in the light of the general teaching of the Scriptures. There is overall harmony in all the books of the Bible, and these two parables should not fall out of this harmony, since the Author, the Holy Spirit, is the same.
The "mustard seed" was the smallest of the seeds known by the people to whom this parable was told. The Kingdom of God, understood as being the so-called Christianity, also began with a small group of people, persecuted and afraid, from which other groups emerged, the primitive churches, which remained relatively pure due to the persecution they suffered.
The bush resulting from the mustard seed was common (it is still grows there) but it never grew to the point of becoming the phenomenal tree of the parable. It was evidently an exaggeration, and this must be taken into account in the interpretation too, because it is part of the teaching.
Christianity in fact began to grow in an abnormal scale when secular power joined it. Its influence around the world became very great, but its quality reduced through its departure from the purity of biblical teaching, with the introduction of heresies, myths, clericalism and ritualism, to the point of prohibiting the reading of the Word of God.
The "birds" of the parable of the Sower, in the interpretation of the Lord Jesus, were of the Evil-one, and came to uproot the message of the Kingdom sown in the heart of those who heard the preaching. They therefore represent something bad.
In this parable, the "birds of the air" come to make their nests in the abnormally large branches of that tree. The same happened to Christianity, which having been inflated by heresies and apostate sects, now harbours a multitude of false "churches", of Christian having only the name. They are instruments of evil to pollute the Christian witness, tares among the wheat as the previous parable teaches us.
The next parable, that of the leaven, is all contained in verse 33. Leaven is the key to the whole chapter, it can also be considered the key to the book of Matthew, and the Gospel of Matthew is the key book of the whole Bible.
The verse tells us what the Kingdom of God is like. There are two substances and a person:
Three measures of flour, i.e., a large amount of flour, because each measure (Greek saton) is estimated to be between 7 and 13 litres. Flour comes from cereal grain, as the seed of the parable of the Sower, where the Lord Jesus explained that the seed represented the Word of God.
Leaven, is always a type of evil in the Bible. When God commanded His people to rid their houses of leaven (Exodus 12:15), they understood this. If anyone ate what was leavened from the first until the seventh day of this Feast of Unleavened Bread, he would be cut off from Israel. Jesus warned against the leaven of the Pharisees and Sadducees (Matthew 16:6,12) and the leaven of Herod (Mark 8:15). In 1Corinthians 5:6-8 leaven is defined as malice and evil, and the context of Galatians 5:9 shows that there it means false teaching. In general, leaven means either evil doctrine or evil behaviour. Reportedly, the rabbinical writers used the leaven regularly as a symbol of evil; therefore, the Jews who heard this parable would understand its meaning, without further explanation.
A woman, in the biblical doctrinal sense, is always a kind of error and evil agent (e.g.. Revelation 2:20). She is forbidden to teach in the church (1 Corinthians 14:34, 1 Timothy 2:12), some have excelled by founding several heretic sects, having taken the place of authorities in doctrine to tamper with the food of the people of God with destructive heresies.
This parable, as the others in this series, is a picture of what is happening with the Kingdom of God and His word in the interval since the rejection of Christ until His exaltation when He comes to take up His kingdom after the Great Tribulation.
Some commentators maintain that the leaven would represent the Gospel and the flour would be the world, where the Gospel would be preached until it fills it. It is a very flawed theory, since the leaven, associated with evil and rot, could never be used as a symbol of the purity of the Gospel. The Gospel is to be proclaimed, and not hidden, furtively, as the woman does in the parable. Furthermore, judging from the experience of two millennia that have almost elapsed, the world will never be saved in its entirety by the Gospel. On the contrary, the Lord Jesus himself said, "When the Son of Man comes, will He really find faith on the earth?" (Luke 18:8) The question indicates that the answer will be negative.
The Apostle Paul refers to this parable in 1 Corinthians 5:6-8, when he talks of the "old" leaven of hypocrisy and falsehood, and of evil and wickedness, and urges believers to celebrate the feast of the Lord's supper with "unleavened bread, the bread of sincerity and truth".
We conclude, therefore, that the three measures of flour represent the Gospel. The quantity (three) probably just means its integrity, although it also reminds us the triune God, who gave us His Word.
The woman picks up the leaven, a symbol of hypocrisy, false teaching and immorality, or sin in general, and places it within the Gospel message, furtively, for no one to notice what she did.
No doubt, it is what we see clearly in our days. From the great ecclesiastical institutions until the multiplicity of sects coming up every day, we see a complete misrepresentation of the message of the Gospel because of false doctrines planted in it. All consider themselves part of "Christianity", and quote parts of the Bible. Even the spiritists and even those who worship the Devil use the Bible.
As a substance, leaven is a living organism or enzyme that causes the transformation of another substance, like dough or grape juice, into something different and with another flavour. Leaven is used to obtain a new flavour, which is more pleasant to the palate, but it is necessary to stop it at some point to avoid total putrefaction.
False teachers of all kinds also put their leaven in the word of God to make it more attractive to many people. However, the parable teaches us that the introduction of false doctrine in the churches will result in apostasy. The Lord Jesus said that at his second coming, He will not find faith on Earth, but there will be general apostasy. That is the way "Christianity" is going and we already see apostasy everywhere.
Writing to Timothy, the Apostle Paul declares that "For the time will come when they will not endure sound doctrine, but according to their own desires, because they have itching ears, they will heap up for themselves teachers"(2 Timothy 4: 3).
We see the final apostasy, completed, painted in the church of Laodicea (Revelation 3:14 -22), which represents the last state of Christianity before the tribulation period, which precedes the second coming of Christ to judge the world and to reign on Earth.
The Lord Jesus told these first four parables to the people in fulfilment of a prophecy found in Psalm 78:2. What He was teaching to the people in parables, were never before revealed truths: they were puzzles, difficult for them to understand.
31 Another parable He put forth to them, saying: "The kingdom of heaven is like a mustard seed, which a man took and sowed in his field,
32 which indeed is the least of all the seeds; but when it is grown it is greater than the herbs and becomes a tree, so that the birds of the air come and nest in its branches."
33 Another parable He spoke to them: "The kingdom of heaven is like leaven, which a woman took and hid in three measures of meal till it was all leavened."
34 All these things Jesus spoke to the multitude in parables; and without a parable He did not speak to them,
35 that it might be fulfilled which was spoken by the prophet, saying: "I WILL OPEN MY MOUTH IN PARABLES; I WILL UTTER THINGS KEPT SECRET FROM THE FOUNDATION OF THE WORLD."
Matthew chapter 13, verses 31 to 35.