Cyril of Alexandria — as excerpted in the Catena Aurea on Luke 13:18-21

“Or else; The kingdom of God is the Gospel, through which we gain the power of reigning with Christ. As then the mustard seed is surpassed in size by the seeds of other herbs, yet so increases as to become the shelter of many birds; so also the life-giving doctrine was at first in the possession only of a few, but afterwards spread itself abroad.”