There are three parts to the text of today. The first is the parable of the mustard seed (13,31-32) then is the parable of the yeast (13,33) and finally the reason why Jesus speaks in parables (13,34-35).
While the parable of the Mustard seed is found also in Mark 4,30-32, Matthew follows the Q version more closely. While in Mark, the mustard seed becomes more correctly a shrub which puts forth large branches (Mk 4,32) and the birds of the air make nests in the shade of the shrub (Mk 4,32), in Matthew, the mustard seed becomes a tree (13,32) and the birds of the air makes nests in its branches (13,32). The tree motif probably has references to the symbol of the imperial tree mentioned in Ezekiel 17,23 and 31,6. The point, however seems to be to contrast the present lowliness of the kingdom with its ultimate greatness.
In the parable of the yeast, we are told about the act of a specific woman in hiding the yeast in three measures of flour, just as the mustard seed had spoken about the act of a specific man in sowing the seed. Yeast, here is used in the positive sense, whereas generally it has negative overtones. The reason for the use of yeast as a symbol for the kingdom is to probably shock the listeners. The quantity of flour into which the yeast is hid is three measures, which would produce enough bread to feed about 150 people, and is indeed a large amount, brings out the aspects abundance and extravagance. The kingdom at present seems small and insignificant, as is the yeast, but it will be revealed in its fullness later.
Though Mt 13,34 parallels the conclusion of Mark’s parable discourse (Mk 4,33-34), which states that Jesus spoke to the crowds only in parables, Matthew has added in 13,35 the eight of his formula or fulfilment quotations. The quotation is from Ps 78,2 and Matthew probably uses it because of the word “parable” found in it, though the context in the Psalm is not about hiding but about revelation.
We might tend to get discouraged sometimes when we cannot see clearly the results of our actions. We have striven hard and at times all that we have to show for our hard work seems negligible in comparison. The parables of the mustard seed and yeast are calling us to continue to sow and mix or in other words to do what is required of us to the best of our ability.