Another parable He put forth to them, saying: “The kingdom of heaven is like a mustard seed, which a man took and sowed in his field, which indeed is the least of all the seeds; but when it is grown it is greater than the herbs and becomes a tree, so that the birds of the air come and nest in its branches.” Matthew 13:31–32
Skeptics claim that Jesus was wrong in saying that the mustard seed was the least of all the seeds, or that He was accommodating the knowledge of His listeners. The late Professor of New Testament Language and Literature W. Harold Mare covered this topic more than adequately when he responded to the critics in a paper published in the Grace Theological Journal (which was published by Grace Theological Seminary from 1960–1991):
Jesus’ statement in Matthew 13:32 about the size of the mustard seed need not, and has no reason to, be interpreted as contradictory to scientific evidence for the following reasons.
In the first place, although, the orchid seed may be the smallest, or one of the smallest plant seeds, and thus smaller than the mustard seed, it is not necessary to consider Jesus’ statement in Matthew 13:32 as containing scientific error since the class of seeds with which the mustard seed is associated is the garden herb group (lachana) which may possibly be interpreted as being the “all the seeds” category to which reference is made in the earlier part of the statement, “all” there being limited to the specific group (lachana) under consideration in the total context of the verse. Since the mustard seed probably was cultivated in Palestine in ancient times, for its oil, it may be argued that Jesus, when speaking of this type of seed, was talking about it in a comparison with all those seeds which were planted by farmers for food. Since panton is used with the lachana group in the parallel passage in Mark 4:31, it may be further argued that the panton ton spermaton group in both Matthew 13:32 and Mark 4:31 is intended to mean only the lachana species, the “all the garden herb” group. In this limited context of garden herbs then, Jesus speaks of the mustard seed as extremely small.
With “all the seeds” being understood as limited in this way by the context, the minute orchid seed need not be considered as being included by Jesus in His statement. It is to be observed that if Jesus had said, “The mustard seed is smaller than the orchid seed,” He would have seemed to have spoken erroneously; but this He did not say.
Secondly, that the expression comparing smallness with the size of mustard seed was a common Jewish saying argues for the fact that scientific literalness and preciseness need not be pressed upon it, it being able to be understood then, as men certainly understand it now, as a general and popular expression of smallness.
However, it is to be realized that Jesus, in using the common Jewish proverbial expression of the mustard seed as a figure of smallness, did so only because the proverbial expression so used was a true and accurate statement, including those implications involving scientific data regarding the mustard seed, both as to its very smallness as a seed and to its moderate largeness when grown.
For those who would like to study this further, Dr. Mare’s entire paper is reprinted here.