The speaker of this poem constantly wonders about what kinds of people will be the last to come to the church after religious belief has faded away. In stanza 4, he wonders about "dubious women" coming to make their children touch part of the church for superstitious reasons, or maybe to pick herbs that might cure cancer. In stanza 5, he asks who the very, very last individual will be to visit the church. Maybe it'll be a builder or carpenter who (like the speaker) is fascinated with the architectural aspects of churches. Or maybe it'll be a person who's completely obsessed with the superficial aspects of the church, like the smell of incense or Christmas. By wondering about these sorts of things, the speaker shows us what religion is when it's stripped of its substance. But what is interesting about these lines is that they imply there is some deeper substance to religion. If the speaker was a total atheist, he'd say that these types of churchgoers are already the same as any normal churchgoer: just superstitious people who are hung up on empty conventions. But through his questions, the speaker implies that the church does have a soul to lose. He just doesn't know what that soul is because he can't bring himself to believe in it personally. That's a real sticky wicket for him.
Line 28: The speaker wonders who will still come to the church when people don't believe in religion anymore. The buildings will look pretty out of place, and this might inspire people to become superstitious and visit the churches thinking they'll bring.
Lines 40-41: When wondering about who the last of the future churchgoers will be, the speaker wonders if it'll be people who are interested in the architectural aspects of the buildings, not the spiritual ones.
Line 42: One of the last visitors might be someone who's just obsessed with old stuff, and not interested in the church's true purpose.
Line 43: This type of churchgoer no doubt existed even in Larkin's time, and here Larkin might be challenging the kinds of churchgoers who might only show up at Christmas because they like the decorative atmosphere.
Line 45: Here, the speaker (and probably Larkin himself) wonders if the last person to visit a church will be like himself, bored yet curious about what the church once meant to people. This line conveys the sense of distance the speaker feels between himself and the type of deep spirituality certain churchgoers might actually experience.
Lines 59-60: In his final lines, the speaker claims that there will always be a churchgoer in the world, even if the physical churches crumble and turn to dust. This is because people will always want some sort of seriousness in their lives, will always want to believe that there is a purpose to human life beyond mere survival.