When Jesus taught parables as prophecies of the course of the church's history until His return, He provided two views of the one subject: specifically, the outward aspect, shown to the multitude of people; and the inward aspect, as revealed to His disciples. He gave the Parable of the Mustard Seed (Matthew 13:31-32; Mark 4:30-32; Luke 13:18-19) to the mixed multitude to disclose evidence of a specific characteristic of the church compared to the outside world.
The historical development of the church of God would be one of humble beginnings. However, this parable contains more than this important truth. Hidden within it is a warning about the perversion of the church's method of growth and of satanic attacks upon it. This parable is an analogy, and as with all analogies, the symbolism is not exact but similar. Therefore, the symbolism of the Kingdom of God being likened to a mustard seed is not identical, yet it explains a particular aspect of the process that the church goes through in preparing for God's Kingdom.
Martin G. Collins
Parables of Matthew 13 (Part Four): The Parable of the Mustard Seed
Other Forerunner Commentary entries containing Luke 13:18: