Jesus gives us two parables, The Mustard Seed and the Leaven, that tell us what God’s kingdom is like. These contain two wonderful promises about the progress the gospel will make. This progress comes in two ways.
Mustard Seed: The Progress of the Gospel in the World
He put another parable before them, saying, “The kingdom of heaven is like a grain of mustard seed that a man took and sowed in his field. It is the smallest of all seeds, but when it has grown it is larger than all the garden plants and becomes a tree, so that the birds of the air come and make nests in its branches” (Matt. 13:31-32).
Here is what the rule of Christ looks like in the world. It is like a tiny seed. Jesus says it is the smallest of seeds. Christ’s kingdom has a small beginning but it will lead to a great and glorious outcome.
Think about the small beginning of the gospel in the world. Our Savior was poor in this world, and he was put to death beside two criminals on a cross. After that, about 120 fearful followers remained, and they met for prayer in an upper room. His first preachers were a few fishermen and publicans who for the most part were unskilled and uneducated men. The first truth that they preached was the cross, which was bound to raise antagonism, since it was a stumbling block to the Jews and foolishness to the Gentiles. The first movements of the faith brought persecution, first on the leaders, and then for the followers. 
Despite its humble beginnings, the gospel spread through the Roman Empire, and then to Europe, India, Africa, Asia, and around the world. What is Jesus Christ doing in the world? He is drawing people to himself.
The Bible gives us a glimpse of what Christ’s kingdom will look like in the future—a “great multitude that no one could number, from every nation, from all tribes and peoples and languages, standing before the throne and before the Lamb, clothed in white robes, with palm branches in their hands, and crying out with a loud voice, ‘Salvation belongs to our God who sits on the throne, and to the Lamb!’” (Rev. 7:9-10).
How is this going to happen? Never be discouraged that a work is small if Jesus Christ is at its center! The kingdom of God is like a tiny mustard seed. It may be small now, but it is part of something glorious that will last forever.
Leaven: The Progress of the Gospel in a Believer
He told them another parable. “The kingdom of heaven is like leaven that a woman took and hid in three measures of flour, till it was all leavened” (Matt. 13:33).
Here is what the kingdom of heaven looks like in the life of an individual believer. If you’ve made your own bread, you know all about this: Leaven, like yeast, produces fermentation in the dough, causing it to rise when it is baked. Every time bread was made, a piece of leavened dough was put into storage. When the next batch of dough was made, the leavened dough was mixed into it, causing the new dough to rise before it was baked.
John Macarthur says, “When a Jewish girl was married, her mother would give her a small piece of leavened dough from a batch baked just before the wedding. From that gift of leaven, the bride would bake bread for her own household throughout her married life.”  Why? Because it’s a living organism. It keeps right on living and spreading.
Jesus is saying that a small amount of the leaven mixed into a batch of dough will permeate the whole lump and change its entire nature. Bake it without leaven and it is hard, crispy, and flat. Bake it with leaven and it rises light and fluffy, and delicious to eat. Think about this in relation to your experience as a Christian. When God’s grace comes into your heart, it is like a little leaven being mixed into the dough of your life.
God began a work of grace in your heart, and gradually what He began in you permeates the whole of your life. God’s grace begins to influence your conscience, your affections, your mind. It is changing your nature from the inside out. Here is what the rule of Christ looks like in the life of a believer: It goes everywhere. No part of the dough is unaffected.
The rule of Christ in the life of a believer may have a small beginning, but it will end in a complete transformation: “He who began a good work in your will bring it to completion at the day of Jesus Christ” (Phil. 1:6).
Jesus Promises Completion
When he appears we shall be like him, because we shall see him as he is (1 Jn. 3:2).
When you become a Christian, you begin to see how far you are from what God is calling you to be. There is a new longing for holiness in your life. Sometimes all you can see is how far short of it you are. Satan keeps saying to you, “Look at what you were.” But Jesus says to you, “Look at what you will be!”
None of us is there yet. Right now we are a mass of contradictions. We love Christ, yet at the same time we feel the pull of the world, and sometimes we behave in ways that grieve Him. We trust Christ, yet at the same time we are subject to all kinds of doubts and fears. We have resurrection life, yet at the same time our bodies are subject to weakness, decay, and death.
But the day is coming when you will love God with all your heart, soul, mind, and strength. You will love your neighbor as yourself. Temptation, pain, and disappointment will be distant memories. Your gifts will be fully deployed in the service of God. God will wipe every tear from your eyes.