Jesus tells people to be obedient and repent so they’ll be ready for the Lord’s coming, illustrating his point with a parable about a fig tree. Then he heals an old woman and a blind man on the Sabbath, which upsets the Pharisees: Luke 12:35–13:21; John 9:1–41
Jesus used the mustard seed to explain the power of the Kingdom of Heaven:
Luke 13:18-19 What is the Kingdom of God like? How can I illustrate it? It is like a tiny mustard seed that a man planted in a garden; it grows and becomes a tree, and the birds make nests in its branches.
Jesus also described faith in a similar way:
Matthew 17:20 I tell you the truth, if you had faith even as small as a mustard seed, you could say to this mountain, ‘Move from here to there,’ and it would move. Nothing would be impossible.
Now, you might argue that mustard isn’t a tree, but under the right conditions, it certainly looks like a tree (scroll down to the last photo on that page) to anyone but a botanist being fussy about plant taxonomy.
At one level, the analogy is a counterpoint to the yeast of the Pharisees and their tricky, misleading teaching. If a little yeast can affect a whole loaf, a tiny mustard seed can become a ‘tree’.
At another level, Jesus is saying to look past how little there is to how much there could be: you don’t need much faith for things to happen (indeed, the faith to ask is enough), and the Kingdom of Heaven doesn’t need a lot of ‘physical’ presence to change the world around it.