“The Kingdom of heaven is like a mustard seed…The Kingdom of heaven is like yeast..” (Mt 13). There is much debate over the meaning of these two short parables. For a more traditional understanding, consider this post.
But some Christians believe that the imagery of the parables is meant to portray the presence of evil within professing Christendom. This is due primarily to an understanding of the Kingdom of Heaven as a “mystery” encompassing Christendom (understood as organized Christianity). Christendom as a whole is understood to contain evil elements mixed with the good, so both parables are usually viewed as picturing that evil. The birds nesting in the mustard tree are unbelievers. It is also pointed out that yeast is often a symbol of evil (Exod 12:15, 19; Matt 16:6, 11–12; 1 Cor 5:6–8; Gal 5:9; but see Lev 7:13–14; 23:17) and so it is asserted that the parable of the yeast portrays the growth of evil within Christendom. This view of the parables is often held in conscious opposition to a view which understands the images of the growth of the Kingdom in the two parables as indicating the ultimate conversion of the world to Christianity before Christ returns.
The Kingdom in Matthew is the rule of God, inaugurated through the words and works of Jesus and consummated at his return. Second, it is very doubtful that straightforward statements that compare the Kingdom of God to yeast or to mustard seed should be understood as a portrayal of evil. After all, it is the growth of God’s rule, not Satan’s, which is being portrayed. One need not assume that birds or yeast must always be viewed as biblical symbols for evil—consider that the imagery of a lion portrays Satan in one context and Jesus in another (1 Pet 5:8; Rev 5:5). The parables of the mustard seed and the yeast speak of the deceptively subtle yet dramatically significant growth of God’s Kingdom. Despite frequent fruitless responses to the Kingdom message, it does bear much fruit in many cases (13:23). Even John the Baptist may doubt its advance, but it is advancing just the same (11:1–6). The strong man is being bound, and his goods are being plundered (12:29).
While some may view this advance of the Kingdom over-optimistically, others view the present age too pessimistically because they do not acknowledge that the Kingdom was inaugurated and began its advance during the earthly ministry of Jesus. It may presently seem as insignificant as a mustard seed, but it will eventually be the largest tree in the garden. Its growth may be as imperceptible as the influence of yeast in a loaf of bread, but in the end it will be pervasive throughout the earth. The use of humble symbols like mustard seeds and yeast is appropriate for God’s humble servant who does not cry out in the streets (12:19) and who rides into Jerusalem on a donkey, not a stallion (21:1–5). The majority of scholars hold that that these parables portray a contrast between the present reality and the ultimate destiny of the Kingdom. That which is now humble will be glorious. The realization that God is already at work and that there is a unity of the ultimate with the present should give all believers hope.