Adam Clarke's Commentary on the Bible

Verse 31 (Matthew 13:31)

The kingdom of heaven is like to a grain of mustard seed - This parable is a representation of the progress of the Gospel in the world; and of the growth of grace in the soul. That grace which leads the soul to the fullness of glory may begin, and often does, in a single good desire - a wish to escape hell, or a desire to enjoy God in heaven.

- Adam Clarke's Commentary on the Bible