[…] sprawlings of flowers, cutFor Sunday, brownish now (4-5)
The fact that the flowers are both sprawling and brown might symbolize the futility of religion. After all, religion tries to conquer the natural world and make it seem like a meaningful place for humans (i.e., God created the animals for man). But even though we try to make the world do what we want it to (i.e., by cutting flowers), our efforts only have a temporary effect, symbolized by the flowers going brown. Also, the sense that these flowers are sprawling demonstrates that, no matter how much religion tries to make the world seem like an orderly place, there is always randomness and chaos that will go beyond our ability to make sense of it.
And let the rest rent-free to rain and sheep (26)
The speaker wonders what will happen to churches after the last of the true religious believers is gone. He muses at first that the places might get turned into museums so people will be able to look at the artifacts with a sort of historical curiosity. But he also wonders if people will just abandon the churches to nature, where rain will eventually wear away the roofs and sheep will walk inside the buildings as they please. This image in particular shows how temporary humanity's time on earth really is. Sure, we can have nice buildings that seem like they'll last forever, but in the end, these buildings (just like human faith) will crumble without constant maintenance. Just as people have to keep going to church every week to keep their faith refreshed, the roof of the church requires constant maintenance.
Grass, weedy pavement, brambles, buttress, sky (35-36)
Again, the speaker talks about how nature has a way of demolishing any human attempts to make the world into an orderly, meaningful place. Further, it also shows that, after religious belief has disappeared from the world, doubt and disbelief will have to disappear, too, since there'll be nothing left for people to doubt. The image of "weedy pavement" is especially provocative for the way it makes you think of a little plant defeating something as strong as pavement by growing up through its cracks. Further, the mention of the "buttress" among the weeds and grass seems out of place, but is very symbolic. A buttress is a human-made support that keeps a roof or tunnel from caving in. The mention of it here suggests that human faith is constantly trying to create buttresses to keep the harsh world from falling in on it. But if faith is gone and weeds are growing inside the church, we find out that all of humanity's buttresses—both physical and spiritual—have ultimately failed.