Jesus tells the parable of the mustard seed in Matthew 13:31-32; Mark 4:30-33, and Luke 13:18-19. The mustard seed is called the smallest of all seed and yet grows larger than the garden plants and provide a nesting place for birds in its branches.
This, Jesus says, is what the Kingdom of God is like.
So what does it mean?
The mustard plant was a very small seed that grows very rapidly (in a matter of weeks) into a decent sized plant—usually around four feet, sometimes perhaps ten feet high or so, and on occasion fifteen feet. It had branches that would be stout enough to hold bird’s nests.
So what’s the relation to the kingdom?
Jesus’ point is not about the size of the kingdom. The mustard seed is not a particularly large plant in comparison to other tree, though it is large in comparison to other herbs (mustard is an herb). While the kingdom of God will be large (as in worldwide) that is not the meaning of this parable. Some suggest that the birds in the branches represent the Gentile nations incorporated into the kingdom. That may be the point of the branches; it may not be.
Jesus’ point is neither about the timing of the kingdom. The parable does not teach that the seed is planted and then slowly grows into an overwhelming kingdom encompassing the whole world during the church age. If anything, it would be the opposite, that the kingdom actually grows very quickly because that is what a mustard plant did. But that is not the point.
The point of the parable is that even though the kingdom of God looks to the disciples as if it is small like a mustard seed, it will assuredly come and be great. The small seed produces a plant out of proportion to its size.
Jesus is using the mustard seed in order to assure his disciples of the success of the kingdom.
To disciples being sent out on mission, and soon to be with out their leader, they needed the encouragement to know that their work was not in vain.
Sure, it seemed small and hard to them. Jesus was being rejected by many. In fact, the parable of the sower which is told immediately before this in Matthew, Mark, and Luke reveals to us that a great many professors will in fact not be true believers. They will fall away for various reasons.
Yet the disciples must not lose heart. Even though their kingdom work might seem to be having little effect, almost insignificant, they were part of a bigger promise.
They must not lose heart.
So it is that the church to whom this parable was recorded in the gospel needs to be reminded of the kingdom promise in times of persecution and difficulty. The church was not being encouraged that the kingdom was gradually coming, or that it was already and not yet (though that may be true in some sense).
The church was being reminded that the mustard seed does grow.
Our labor is not in vain, though it may appear hard and even hopeless at times.
The kingdom will come. Jesus will reign. It will be worth it all.