Why does a mustard seed attract comparison to the kingdom of God? Because for Jesus the kingdom is about the ordinariness of loving God and loving others. The kingdom is as common as sparrows, as earthy as backyard bushes, as routine as breakfast coffee, and as normal as aging. He hallows the ordinary act of love, making it extraordinary. Instead of finding it in the majestic, Jesus sees God's kingdom in the mundane. The kingdom of God is the transforming presence of God in ordinary human beings who live out the Jesus Creed.
Scot McKnight, The Jesus Creed: Loving God, Loving Others, 135, 136.