Parable of the Mustard Seed: Matthew 13:31-32; Mark 4:30-32; Luke 13:18-19
The parable of the mustard seed is one of several parables that Jesus uses to describe the kingdom of God.
In this parable of the mustard seed, Jesus make the statement that the mustard seed, "is the smallest of all seeds on earth" (Mark 4:31), and that "it grows up and becomes the greatest of all shrubs" (Mark 4:32), even as big as a tree.
There are those who criticize Jesus for making such statements because if Jesus Christ is God then surely he would know that there are smaller seeds in the world (a few plant varieties have slightly smaller seeds). However, these critics fail to understand that Jesus speaks to people in terms that they can understand. Otherwise, how can communication occur if the audience does not understand?
To Jesus' audience, the smallest known seed would be a mustard seed, which can in fact grow to be a plant as large as a tree.
In addition, Jesus Speaks in Figures of Speech. Saying that this mustard seed is the smallest seed or that it grows into the biggest of shrugs may be exaggeration used to emphasis a point.
Jesus exaggerated in many situations, such as when he tells the Pharisees, "You hypocrite, first take the log out of your own eye, and then you will see clearly to take the speck out of your neighbor's eye." (Matthew 7:5) Or when Jesus tells Peter to forgive, "Not seven times, but, I tell you, seventy-seven times." (Matthew 18:22) Jesus is making the point to forgive as many times as necessary.
A related article is Is the Mustard Seed the Smallest of Seeds? (opens in new window)
Reading this parable in context, Jesus' point is not to literally tell us what the smallest seed on earth is. Instead, the reason Jesus is telling this parable is to communicate what the kingdom of God is like, by using a metaphor that his audience could relate to.
The truth Jesus is communicating about the kingdom of God is that the influence it has begins like the smallest of seeds, yet it grows to be like the largest of plants. This truth can be seen on the individual, communal, and global levels.
In an individual, the reign of God grows until that person becomes completely obedient to God. In the community, the reign of God can spread from person to person as they teach each other to "know the Lord." (Jeremiah 31:34) In the world, the reign of God began with Jesus Christ like a tiny seed that has grown into a large group of believers obeying God.
The Parable of the Yeast has essentially the same meaning as this parable of the mustard seed.