Background
Jesus gave the parable of hidden leaven to the multitudes who were following him (Matthew 13:34). He initially taught it between October 28 A.D. and Early 29 A.D. He later gave it a second time, between Mid-November and Late December 29 A.D. (Luke 13:20 - 21).
The hidden leaven parable of Matthew 13 was taught to a multitude of people who heard Jesus as he preached from a boat while they were on the shore of Galilee's sea (Matthew 13:1 - 2). The three parables which preceded it, the Sower and the seed, tares and mustard seed, were also taught to the public.
This parable centered on leaven is the fourth of eight in Matthew 13 given to reveal characteristics related to the kingdom of heaven. This kingdom, recorded eight times in the King James version of the chapter (verses 11, 24, 31, 33, 44, 45, 47 and 52), is the same as the Kingdom of God taught by the other gospel writers.
The Parable
Another parable He spoke to them: "The kingdom of heaven is compared to leaven which a woman took and hid in three measures of flour until all was leavened." (Matthew 13:33).
The positive use of leaven in Matthew 13 and Luke 13 is rare in the New Testament. This is because it usually symbolizes a bad, corrupting influence (see Matthew 16:6, 11 - 12, Mark 8:15, Luke 12:1, 1Corinthians 5:6 - 8, Galatians 5:5 - 9). The leaven's positive use is prophetic as it predicts the kingdom's ultimate spread to everyone wherever they exist!
What Is It?
Leaven is a substance that is "hidden" within flour that, through fermentation, works subtly and slowly, but progressively, to permeate and change its fundamental structure. When flour with leaven is baked it produces bread that has significantly more volume than the original flour.
In Jesus parable, the leaven added to three measures (roughly a bushel) of flour would have produced enough bread to feed about 100 people (Bible Background Commentary)!
The Meaning
Jesus used leaven within flour to reveal that although the Kingdom of Heaven (God) starts out small and hidden, it will unstoppably grow to become massive.
The Kingdom of God began with Jesus, its King, preaching the gospel and calling all people to repent (Matthew 3:2). He then chose twelve disciples, trained them in evangelism, and sent them out to preach. After his resurrection many others, like the Apostle Paul, were called to spread God's truth even farther.
There have been many people called and converted since the first century (and some converted before this period like Abraham, David, etc.). This group, however, is still incredibly tiny compared to the estimated 117 BILLION people that have so far been born on earth (Population Reference Bureau)!
The Leaven Spreads!
The group of people brought back to life in the first resurrection (Revelation 11:10 - 12, 1Thessalonians 4:16 - 17, 1Corinthians 15:51 - 53), when Jesus returns, will still be relatively few (Matthew 20:16, 22:14). These people, however, will be used by the Lord to continue the expansion of God's kingdom as symbolized in the parable of leaven.
The resurrected saints will play active roles in Jesus establishing God's kingdom on earth during the Millennium. They will help in the salvation of many others during 1,000 years of peace and prosperity. Those given immortal life at Jesus' Second Coming, and those who receive salvation during his rule, will then combine to expand the kingdom even further!
A now much larger set of immortal spirit beings will be available to help the BILLIONS of people brought back to life in the second resurrection.
Those resurrected after the Millennium, at the Great White Throne Judgment (Revelation 20:12), are all those who ever lived (including those aborted) from Adam to Jesus' return who never had a full chance at salvation. God will open their minds to understand the truth, after which they will be helped to achieve their full potential by those who already received eternal life.
Conclusion
The parable of hidden leaven reveals the goal of God's kingdom is to ultimately reach all humans. Those who believe and live by its teaching will be eternally blessed (Revelation 21:24 - 27).