The mustard seed stands for the progress of the church from small beginnings. Because of its minuteness, the mustard seed came to symbolize small beginnings, denoting the smallest weight or measure, a tiny particle. The parable focuses on this idea of smallness. The mustard seed is something small that does its part to expand in preparation for the Kingdom of God. The seed represents an instrument by which spiritual growth can be advanced, just as a plant grows and reproduces itself through a seed.
In this parable, the small seed is the church, which appeared as the firstfruits of the Word. Just as in the Parable of the Sower, the one who sows the mustard seed is the Son of Man, Jesus Christ, and the field is the world. Jesus Himself had an insignificant entrance into the world by human standards, and the church He founded is likewise a "little flock" (Luke 12:31-32), small and designed by God not to become a physically powerful organization that would make a spectacle of itself.
In Matthew 7:13-14, Christ says the way that leads to eternal life is difficult and narrow, and few find it. He reiterates in Matthew 20:16 that few are chosen. In Luke 10:2, when sending the seventy out, He says the laborers are few. Paul argues in I Corinthians 1:26-29 that God calls the weak and the base of the world to put to shame the mighty and the noble. Jesus is referring to those few who, upon their calling by God, voluntarily submit to God's dominion, the Kingdom of God.