Some argue that the Bible is contradictory concerning whether the mustard seed is the smallest seed. Matthew 13:31 says that it is, which seems incorrect. However, there is a simple answer to this alleged contradiction in the Bible.
He put another parable before them, saying, “The kingdom of heaven is like a grain of mustard seed that a man took and sowed in his field.Matthew 13:31
And he said, “With what can we compare the kingdom of God, or what parable shall we use for it?Mark 4:30
It is like a grain of mustard seed that a man took and sowed in his garden, and it grew and became a tree, and the birds of the air made nests in its branches.”Luke 13:19
The purpose of Matthew 13:31, Mark 4:30, and Luke 13:19 was to teach a lesson using an analogy, not to make a technical statement concerning the size of a mustard seed. If Jesus’ purpose in saying that the mustard seed is the smallest of all seeds was to state a scientific, technical fact, then the Bible would indeed contain an error.
However, if Jesus’ purpose was to speak proverbially and to teach a lesson using an analogy, then there is simply nothing wrong with these passages. Jesus’ intention was to speak in a generic sense to convey a particular message.
There are some foundational principles that apply to all alleged and apparent contradictions in the Bible. To read more, see Bible Difficulties: Foundational Principles.
To read more answers to alleged and apparent contradictions in the Bible, see “Contradictions” in the Bible Answered.
These books are also excellent resources: