Why did Larkin call the church "a serious house" in "Church Going"?

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."