From The Gospel of Matthew, by Curtis Mitch and Edward Sri, commenting on Matthew 13:24-43
The parable of the mustard seed builds on the previous one, showing that despite the enemy’s opposition to the kingdom, the harvest will yield tremendous results.
At first, the kingdom does not appear to be very large. It is like a mustard seed, which was proverbially the smallest of all the seeds. From this tiny seed, a great bush emerges. Jesus describes it as becoming so big that birds of the sky come and dwell in its branches—which recalls an Old Testament description of a great kingdom that gathers many nations as a large tree gathers birds who nest on its branches (Ezek 31:2–13; Dan 4:17–18).
In particular, Ezekiel foretold that Israel would gather the nations like a mighty cedar that shelters the birds of the air (Ezek 17:22–24). Jesus uses this parable to show how his kingdom movement, despite its small beginnings, will become like the prophetic large tree gathering birds, fulfilling Israel’s mission to the nations as Ezek 17 foretold.
….Commenting on the Church’s evangelizing efforts, Pope Benedict XVI warned that Catholics today must resist what he calls “the temptation of impatience,” that is, the temptation to insist on “immediately finding great success” and “large numbers.”
He says that immediate, massive growth is not God’s way. “For the Kingdom of God as well as for evangelization, the instrument and vehicle of the Kingdom of God, the parable of the grain of mustard seed, is always valid.”
He goes on to say the new phase of the Church’s evangelizing mission to the secular world will not be “immediately attracting the large masses that have distanced themselves from the Church by using new and more refined methods.” Rather, it will mean “to dare, once again and with the humility of the small grain, to leave up to God the when and how it will grow.”