Expositor's Bible Commentary Commentary Mark 4:31

"It is like a grain of mustard seed, which, when it is sown upon the earth, though it be less than all the seeds that are upon the earth," — Mark 4:31 (ASV)

This is the third and last of the parables about the seed sown. The mustard seed was proverbial for its smallness but when grown, it becomes a huge treelike shrub.

The main point of the parable is that the kingdom of God has insignificant and weak beginnings, but a day will come when it will be great and powerful. We should not judge the significance of the kingdom by the size of its beginning.