(31) 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, (32) 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."
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Aside from this parable, His only other mention of a mustard seed relates to faith (Matthew 17:20; Luke 17:6). This usage does not mean that the mustard seed is faith, but faith is nevertheless a component of the growth or increase to which Jesus refers in the symbol of the seed. In addition, He describes the mustard seed as “the least of all the seeds,” linking it with God's description of Israel as “the least of all peoples” (Deuteronomy 7:7). Its first human king, Saul, even protested that he was from the smallest tribe, and his family was “the least of all the families” (I Samuel 9:21).
The real beginning of the nation/kingdom, though, was Abraham. Through Abraham's faith, the nation (which became a kingdom) began. By his faith in God's promise of an heir, the nation grew. God promised Abraham that “kings shall come from you” (Genesis 17:6) and that “kings of peoples shall be from” his wife Sarah (verse 16). Thus, the “least of all peoples” had a faithful seed (beginning), and this seed likewise grew because of faith.
The commentaries are divided over the nature of the mustard plant in this parable. Some suggest that something contrary to nature takes place for the mustard plant to be considered a tree large enough to support birds. Yet others assert that the mustard plant can reach fifteen feet in height and provide anecdotes that suggest Jesus describes nothing unusual in His parable. Even today, mustard plants in the area of modern Israel are found with branches an inch in diameter. Relative to other local herbs such as hyssop, as well as the minuscule seed involved, a one-inch branch is massive!
Historians often herald Solomon's kingdom as the Golden Age of Israel, yet it was also oppressive and unsustainable. Ultimately, his wives turned his heart from God, and the worship of foreign gods (demons) received official sanction within the “thriving” kingdom. God blessed Solomon's kingdom, yet through his unwise excesses, it ultimately veered in a disastrous direction.
Because Jesus does not explain the mustard tree's size either way, it proves helpful to consider the elements of the parable that remain. First, Jesus draws attention to the fantastically humble beginning, which applies to Israel. Second, He points out its tremendous growth relative to its minuscule beginning. However, even with this surprising growth, the nation of Israel did not overshadow other “trees”—other kingdoms. Instead, it “grows up and becomes greater than all herbs” (Mark 4:32).
A third element is that the final state of the mustard tree is as a host to birds. This third point is central because Jesus uses birds as a symbol for Satan and his demons (Matthew 13:19). The humble mustard plant, with its faithful beginning and dramatic growth, in time became a place where the demons felt at home.
The gospels highlight demon possession as a major problem in Judea and Galilee during Christ's short ministry, and casting out demons was a significant part of His and the disciples' work. Because of Israel's unfaithfulness, God had removed His protection, and demons were “nesting” everywhere in the kingdom. The parable describes the nation's then-current satanic state rather than, as many commentators hold, the growth of the then-future church.
God inspired Moses to write that when Israel grew large through His increase, she would also fall into idolatry (Deuteronomy 32:15-17), which involves demonism, an exact parallel to what Jesus describes in the Parable of the Mustard Seed. Moses knew that Israel would “become utterly corrupt,” warning them that “evil will befall you in the latter days, because you will do evil in the sight of the LORD, to provoke Him to anger through the work of your hands” (Deuteronomy 31:29). Israel's corruption is a consistent Old Testament theme (Psalm 14:3; 53:3; Isaiah 1:21; Jeremiah 7:11; 10:21; Ezekiel 16:47; 23:11). Without the new heart and Spirit available under the New Covenant, she followed the world's course into spiritual uncleanness and demonic activity.
— David C. Grabbe