Matt. 13:31-2 Another parable put he forth unto them, saying, The kingdom of heaven is like to a grain of mustard seed, which a man took, and sowed in his field: Which indeed is the least of all seeds: but when it is grown, it is the greatest among herbs, and becometh a tree, so that the birds of the air come and lodge in the branches thereof.
It is argued that Jesus was wrong here because there are smaller seeds in existence, like the orchid.
But the Greek word here is mikros, and it does not merely have size in view. For example, it is also used in Luke 9:48 -
And said unto them, Whosoever shall receive this child in my name receiveth me: and whosoever shall receive me receiveth him that sent me: for he that is least among you all, the same shall be great.
Obviously, Jesus did not mean here that he who was smallest in size would be greatest. So we suggest that the mustard seed description is not an evaluation of size at all (or exclusively), but an evaluation of worth - which is a matter of personal judgment that is quite acceptable and cannot be charged as erroneous.
Two other objections may crop out of this one. First, some versions refer to the mustard as a "shrub" but the word here is lachanon, meaning a vegetable or an herb. (Rom. 14:2 "For one believeth that he may eat all things: another, who is weak, eateth herbs.")
A second objection is that mustard becomes a tree. Of course, the scientific classification schemes of modern botany were not yet invented -- classification was by appearance and function, not by biology. In that regard, the mustard plant of Palestine (Sinapis nigra) could grow to a height of several (2-6) feet and be considered a tree -- if not by the scheme of modern science (Is a bonsai a "tree"?), then assuredly by the descriptive classification scheme of the ancients.
-JPH
Update June 2026: I was made aware of a "reply" to the above on the so-called RationalWiki site ("Serving 15 year old atheists amateur slop since 2007"). The idiocy offered is as follows:
It is difficult to make sense of the suggested translation: in what sense is mustard the "least" of seeds, compared to, say, the seeds of weeds? How is the seed useless if it grows into something great?
The 15 year old responsible for this reply apparently is unaware of their failure, of a type repeatedly called down here: The contextualizing frame of honor and shame. When the movement started by Jesus began, it had no honor in its social setting. It was not ascribed any honor, nor did it have any characteristics that others would regarded as honorable. It was a movement founded by someone (Jesus) who would have been regarded as deviant. It is in that sense that the evaluation is one of worth rather than size. In contrast, an analogy of size makes no sense at all. The kingdom of heaven is not a physical object that you can apply a measuring device to. The analogy refers rather to the accrual of honor which would be obtained through the validating event of Jesus' resurrection and the movement's gradual rise to prominence. This sort of idea, of course, is too advanced for the average 15 year old atheist to grasp.
Beyond this, the 15 year old stumbles through a Strong's concordance which assures him that lachonon absolutely can mean "shrub". From this the teenager concludes I was conspiring to hide this meaning. The prime idiocy of this statement is found in the teenager's confusion of categories: the category "shrub" is relatively modern and it takes a profound idiocy to think that the Greeks had a specific word that equalled the modern category, "perennial woody plant." In contrast, the Greeks certainly would have had words for basic food types like "herb" and "vegetable". Note furthermore that the same word is used in Luke 11:42 to say that the Pharisees tithede mint and rue and "all manner of herbs". Perhaps Strong's will help the 15 year old to explain why the Pharisees might have been interested in tithing a woody perennial, and also explain how mint and rue fit in that category rather than being herbs. Finally, I would add that the online version of Strong's found here says it means a garden herb or vegetable.
The 15 year old further whines that I use a "it was right back then" approach to defining what a tree is and pretends that Jesus being God would use a modern classfication scheme to express what a "tree" was to 1st century peasants. The profound idiocy of that particular requires no further comment.