William Burkitt on Matthew 13:31

Our Saviour's design in this parable is, to shew how the gospel, from small and little, from unlikely and contemptible beginnings, shall spread and increase, fructify and grow up; like a mustard seed, one of the smallest grains, grows up to a considerable tallness; and as a little leaven turns a great heap of meal into its own nature; so the gospel shall spread and increase, nations and countries becoming Christians.

Learn, That how small beginning soever the gospel had in its first plantation, yet, by the fructifying blessing of God, it has had and shall have a wonderful increase.