Or, after dark, will dubious women come To make their children touch a particular stone; Pick simples for a cancer; or on some Advised night see walking a dead one?
Right away, stanza 4 carries on from the thought introduced in line 27, where the speaker wonders if churches will be thought of superstitiously in the future, the same way we might think of getting bad luck from breaking a mirror. The speaker paints a vivid picture of how people might act in this future world, with "dubious" (of a questionable, or doubtful nature) women bringing their children to touch a certain part of the church just for good luck.
(In this case, Larkin highlights a difference between religion and superstition. Superstition usually has to do with good or bad luck, or with wanting good things to happen to you. True, some religious people might pray for good things to happen too, but what makes religion different from superstition is the larger sense of significance that religion tries to give to people's lives. Here, though, the speaker is suggesting that future-churches will remain ground zero for superstitious behavior.)
On this same note, the speaker wonders if people might come and "Pick simples for a cancer," meaning that they might pick herbs growing out of the church in order to heal someone. Treating the church this way represents the decay of all the specific morals and meanings the church once stood for.
The speaker muses humorously on the thought of zombies walking around the church, which turns the church from something solemn and dignified into the setting for a monster movie. Or perhaps he means to suggest that, on a given ("advised") night, ghosts might appear.
Either way, these future churches seem to be no better than haunted houses (no offense, to haunted houses, you understand).
Power of some sort or other will go on In games, in riddles, seemingly at random;
These lines really sum up the speaker's discussion of religion giving way to superstition, suggesting that, even if religion completely does go away, people will continue to invest in the idea of supernatural power, even if it's just in "in games, in riddles, [and] seemingly at random" (33).
Again, the speaker sketches the difference between superstition and religion. Superstitious powers usually work "seemingly at random" (33), while the church is supposed to do just the opposite, giving a sense of consistency, order, and permanence to the world. In the future, though, religious order will give way to superstitious randomness. That, and riddles.
But superstition, like belief must die, And what remains when disbelief has gone? Grass, weedy pavement, brambles, buttress, sky,
But eventually, Larkin's speaker suggests that even superstition will eventually have to die away, too.
Not only that, but disbelief will have to die with it. It might not be clear at first, but Larkin uses the word disbelief here because he's actually talking about the skeptics and doubters who think religion and superstition are dumb.
The thing is, though, that once these people get their way and religion disappears, there'll be nothing left for them to disbelieve. They'll disappear from the Earth, too.
And what happens then? There'll be nothing of the church building left to contemplate except "Grass, weedy pavement, brambles, buttress, sky" (36). In other words, the natural world will be all that remains, without any higher sense of purpose (or even a sense of doubt) for humans. It'll just be one big… nothing.