The Catechism in Questions & Answers

152. How did Jesus explain the beginning and the growth of the kingdom of God?

Jesus explained this in the parable of the mustard seed. Thereby he showed the humble beginnings of the kingdom of God as well as its development and growth.

“‘The kingdom of heaven is like a mustard seed [...] which indeed is the least of all the seeds, but when it is grown it is greater than the herbs and becomes a tree, so that the birds of the air come and nest in its branches. Another parable He spoke to them: ‘The kingdom of heaven is like leaven, which a woman took and hid in three measures of meal till it was all leavened’.”
Matthew 13: 31-33