“What shall we say the kingdom of God is like, or what parable shall we use to describe it? It is like a mustard seed, which is the smallest of all seeds on earth. Yet when planted, it grows and becomes the largest of all garden plants, with such big branches that the birds can perch in its shade.” (Mark 4:30-32)
A mustard seed is tiny it’s about 1mm thick and from this tiny seed a mustard plant would grow often reaching well over 6ft in one year and would attract lots of birds who would come and perch in its branches and eat its seeds. Have you ever wondered why Jesus compared the kingdom of God to a mustard seed?
The mustard seed – one of the smallest seeds that grow into ‘the greatest of all shrubs, and puts forth large branches. The image should make us stop and think, perhaps Jesus is being slightly satirical. I would think a more appropriate image would be a mighty oak tree growing from a small acorn. Or perhaps a more biblical tree, like a mighty cedar tree, in fact in Ezekiel in the old testament it says:
God says ‘I myself will take a shoot from the very top of a cedar and plant it … it will produce branches and bear fruit and become a splendid cedar. Birds of every kind will nest in it; they will find shelter in the shade of its branches’.
But here Jesus doesn’t compare the kingdom of God to the tallest and strongest of trees. Jesus likens the kingdom of God, the church to something that sprouts up quite quickly from almost nothing and the develops into an ungainly spindly shrub. This should make us smile, Jesus is giving us a humorous picture of the kingdom of God that contains a deep meaning.
Churches I think can take comfort from the lips of Jesus. Like the mustard plant, a church can be an untidy sprawling shrub. But Jesus is saying something quite profound about the church; It will be a bit a messy and jumbled but in the mess is real life, and perhaps it isn’t easy to find your place in neat and tidy systems. But in Jesus’ church, that is a bit messy and tangled, there is a place and room for everyone (Martyn Percy).
Bitesize Bible 
I dont think Jesus was joking . He was trying to teach the people just a little bit of faith can give amazing results
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According to the context, what I understand is that Jesus had already given a full picture of what the kingdom of the heaveans is like on the positive side, from the seed in the good soil, growth, bearing fruit, ripen, harvest and into the barn! That´s it!
Then He starts showing the negative side of what appears to be the kingdom but is not. So you have the tares, the mustard seed and the leaven. The mustard seed has amazing growth in nature, but does not produce a ripe crop of wheat, but a shrub, something where the evil birds (mentioned before) come to rest in.
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