A grain of mustard seed is man's good before he becomes spiritual, which is the least of all seeds, because he thinks to do good of himself, and what is of himself is nothing but evil; yet whereas he is in a state of regeneration, there is something of good, but it is the least of all things; at length, as faith is conjoined with love, it becomes greater, and an herb; and, lastly, when the conjunction is perfected, it becomes a tree; and then the birds of the heavens, which in this passage signify truths or things intellectual, build their nests in its branches which represent worldly knowledge.

(References: Arcana Coelestia 55; Matthew 8:31, 8:32)