Luke 13:18-21: The Parable of the Mustard Seed and the Yeast.
These two parables are talking about two sides of the same coin. The mustard seed speaks of the outward growth of the gospel of Jesus. We Christians only need to go outside and plant it in the lives of the people. Simple as that. The Lord will take care of the growth and the expansion. Our job is to plant, that is, to speak and live out the Word; His job is to make it grow.
The parable of the yeast speaks of the inward growth of the gospel in our lives. We Christians only need to read our Bibles and pray everyday. We only need to incorporate the truths of the Bible to our daily lives. Then the Lord will take care of the inward growth. He will increase our faith and bring us to maturity without even us knowing how.
The problem starts when we forsake our quiet time with God and start worrying about how our ministries will grow. We look for ways to expand, for methods to multiply and reach out the big world out there in exchange of our inner lives. These are not the ways of the Kingdom of God.