[…] wondering, too, When churches fall completely out of use What shall we turn them into (21-23)
Notice in this passage that the speaker doesn't say "if" churches fall out of use, but "when" they do. He therefore treats the decline of churches as unavoidable. He does this by suggesting (though never directly saying) that time is always around us, endlessly slipping away, and though it might not happen in the near future, all human institutions will one day be swept away by the ravaging force of time.
Superstition, like belief, must die,And what remains when disbelief has gone? (34-35)
The phrase "must die" again reflects the inevitability of time passing. And with this inevitability comes the fact that everything that exists will one day not exist, and this is the future era that the speaker is wondering about. On the one hand, he might actually be interested in playing out these future scenarios. But on the other hand, he also constantly implies that just as religious faith will eventually disappear from Earth, so will humans. What is fascinating about this is that the fact that time actually destroys the same thing that it creates. In other words, time sweeps away all of our attempts to create something permanent and human-made, but the temporary nature of human life is also what causes us to seek out this higher sense of permanence. What time takes away with one hand, it gives with the other. For Larkin, it might be both the creator and destroyer of faith at the same time? How's that for mind-bending?
A shape less recognizable each week, A purpose more obscure (37-38)
As time passes, we see the church beginning to decay in both a physical and symbolic way. As it decays, we can almost see its edges getting eroded by the rain, the roof collapsing, the spire tumbling over, until we no longer know what we're even looking at. Well this is exactly the same thing that will happen with people's religious faith as the passing of time continues to erode religious belief in everyday life. This will eventually take religion to the point where people can just barely remember what it was supposed to be in the past. By this point though, its true purpose will be forgotten.