See the notes at Matthew 13:31-32.

A grain of mustard seed - See on Matthew 13:31, Matthew 13:32 (note).

It is like a grain of mustard seed,.... That is, the kingdom of God spoken of in the preceding verse, is like unto a grain of mustard seed; by which is meant, either the Gospel, or the Gospel church state, or the grace of God in the hearts of his people, and it may include them all: the Gospel is so called, because it treats of the two latter; but more especially, because it brings life and immortality to light, or points to the kingdom of heaven, directs the way unto it, and shows what qualifies persons for it, and gives them a claim unto it: and the Gospel church state may be so called, because here Christ dwells, and rules as king; the members of it are his subjects, and the ordinances of it are his laws, to which they are obedient: and the grace of God in the hearts of his people may be so called, because it is a governing principle in them; it reigns through righteousness unto eternal life, and by it Christ reigns in and over them: now the kingdom of God in each of these senses, may be compared to a grain of mustard seed, for the smallness of it, as follows;

which when it is sown in the earth, is less than all the seeds that be in the earth. The Gospel was first preached by very few persons, and these of no figure and account, especially at their first setting out. John the Baptist came preaching the kingdom of God, clothed with a garment of camel's hair, and with a leathern girdle about his loins; our Lord himself made no pompous appearance, there was no form nor comeliness in him; he was a man of sorrows, and acquainted with griefs, and of a mean descent and occupation; his disciples were fishermen, and illiterate persons; those to whom it was preached, and by whom it was received at first were but few, and these were the poor and the unlearned, and publicans and sinners. The Gospel church state at first, consisted of very few persons, of Christ and his twelve apostles; and at his death, the number of the disciples at Jerusalem, men and women, were but an hundred and twenty; the several Gospel churches formed in the Gentile world, rose from small beginnings; from the conversion of a very few persons, and these the filth of the world, and the offscouring of all things. The grace of God in the hearts of his people at first, is very little; it can scarcely be discerned by themselves, and is ready to be despised by others; their light and knowledge, their faith and experience being so exceeding small.

When it is sown (ὅταν σπαρῇ)

This phrase is repeated in Mark 4:32. Here the emphasis is on ὅταν, when. It is small at the time when it is sown. In Mark 4:32 the emphasis is on σπαρῇ, it is sown. It begins to grow great from the time when it is sown.

That are upon the earth

A little detail peculiar to Mark.

It is like a grain of mustard seed, which, when it is sown in the earth, is less than all the seeds that be in the earth:

4:31 Like a grain of mustard seed. See note on Mt 13:31-35. Compare Lu 13:18-19.

4:21-34 These declarations were intended to call the attention of the disciples to the word of Christ. By his thus instructing them, they were made able to instruct others; as candles are lighted, not to be covered, but to be placed on a candlestick, that they may give light to a room. This parable of the good seed, shows the manner in which the kingdom of God makes progress in the world. Let but the word of Christ have the place it ought to have in a soul, and it will show itself in a good conversation. It grows gradually: first the blade; then the ear; after that the full corn in the ear. When it is sprung up, it will go forward. The work of grace in the soul is, at first, but the day of small things; yet it has mighty products even now, while it is in its growth; but what will there be when it is perfected in heaven!