The reading will be Luke 13:28-31, and I'm sure you will be familiar with it... the mustard seed grows into a great tree. The yeast leavens a whole measure of flour. What is the point? The comparison....
Jesus introduced these two illustrations with these words: "With what can I compare the kingdom of God?"
The kingdom looks humble, weak, even worthless from our perspective. Just like a tiny mustard seed amongst a thousand others in a bag. Just like a tiny ball of inert yeast in its container. But when they are given life, watch them work!
So too is the kingdom of God - it is living and vibrant, as yet unseen to the inhabitants of this world, and yet will be fully realised and known in power on the Day it is revealed to all people who have ever lived - the Day of the Lord.
Maranatha (Come, Lord Jesus!).