The Parable of the Mustard Seed is found in Matthew 13:31-32 (RSV):

"The kingdom of heaven is like a grain of mustard seed which a man took and sowed in his field; it is the smallest of all seeds, but when it has grown it is the greatest of shrubs and becomes a tree, so that the birds of the air come and make nests in its branches."

This parable illustrates how the kingdom of God starts small but grows significantly. Though it may seem insignificant at first (like a mustard seed), it will expand and influence the entire world. This reflects the growth of Christianity from Jesus and His small group of disciples to a global faith.