Larkin refers to the church that he has visited in the final stanza of this poem as a "serious house" because he, although he is an atheist, recognises the solemnity of the church and its serious aspect. It is important to realise that this poem talks about the way that churches will always have a role for humans, even when organised religion has "passed away," as Larkin muses about earlier on in the poem. Whatever the future of Christianity, however, Larkin believes that churches will always have a role, precisely because churches are "serious houses" on a "serious earth." We often surprise ourselves with a "hunger to be more serious," and churches are the perfect places to satisfy that hunger, as in them "all our compulsions meet." Larkin comments that this aspect of churches "can never be obsolete." There is something that transcends human experience that can be found in churches, he argues, that will always serve a role for the human species. This is why the church is described as being "serious."
The statement that the church is a "serious house" is a production the atheistic narrator of the poem. Though he lacks belief in religion, he removes his cycle-clips in "awkward reverence", and reflects how "dubious women" might visit the churches to acquire religious power to heal diseases, though superstition and disbelief might not control human life when the churches have fallen "completely out of use." In the sixth stanza, he thinks of the fact that church plays an important role in a man's life: its presence is seen in the birth of a child, in joining two partners in marriage, and in death, a man's end of life. Thus, he comes to the conclusion that church is a "serious house" not because of the standard religious practices, but because of the fact that there is something unexplained about churches which is inevitably connected to the deep truths about human life.