“To what shall we compare the Kingdom of God, or what parable can we use for it? It is like a mustard seed that, when it is sown in the ground, is the smallest of all the seeds on the earth. But once it is sown, it springs up and becomes the largest of plants and puts forth large branches, so that the birds of the sky can dwell in its shade.”
The critic will point out that the mustard seed is a low-lying bush.It doesn't become the "largest of plants," nor does it "put forth large branches...." Jesus is using typical Hebraic exaggeration when he calls it "the largest of plants." His disciples understood that and didn't hesitate to repeat it for the benefit of a thousand generations to come.
Moving beyond that carping, let's notice how the "birds of the air can dwell in its shade." Small birds do, in fact, hide in the bushes, invisible and out of reach to the prowling raptors. Backyard bird watchers are advised to place bird feeders close to dense bushes, lest the hawks and owls catch them in the open. That may be more sport than their three-old granddaughters are prepared to see!
"His eye is on the sparrow" and the the Kingdom of God provides shelter for his little ones, those who donate small copper coins and play their drums for him.
I don't know if the mustard seed is a nuisance bush like many invasive species, especially the buckthorn, in the United States. But I'll take a preacher's liberty and suppose the Kingdom of God is also like an invasive species. It creeps into overlooked places like prisons, nursing homes, military barracks and neglected neighborhoods while political and economic "raptors" are looking elsewhere. They might rid the universities, government and major corporations of the Gospel but they can't be bothered with the hovels of the poor. When they notice its abundance they dismiss it as the worthless succor of the insignificant.
The clever buckthorn spreads in two ways. Animals eat its succulent berries and distribute seeds throughout the neighborhood as the indigestible pits pass through them. And its roots spread through the ground, resurfacing at some distance from the mother plant. Ridding one's wooded copse of these plants takes years of persistent labor.
Likewise, the Kingdom of God spreads through the people who welcome its message. Their peaceful, confident generosity invites the disheartened and the lonely to ask the reason for their hope. They need only an invitation, "Come and see!"
Pope Benedict optimistically saw how secularized Europe retains its Christian culture with thousands of churches, shrines and monasteries. Even the ruins speak of peace. Rooted in history and culture, the very stones announce the gospel to later generations.
These seem like hard times for the Church and the Kingdom of God. Economic prosperity offered opportunities for secular and heretical "gospels." Many people believed they could have all this and heaven too!
But the Gospel has always thrived in hard times, among dispossessed and despised people. It takes root and deepens through droughts, fires, and floods. Even as preying raptors soar aloft, praying sparrows find shelter in its thick, impenetrable branches.