"A serious house on serious Earth it is." What is "it" referring to in "Church Going"?

The reference you have given is the first line of the last stanza,
and refers to the importance of the church, both to Larkin and to society at large.
Having visited a church and being surprised at the solemnity and power that it still
has, this line seems to cement this conclusion with the repetition of the word "serious"
to indicate that, whatever our religious beliefs, the church acts as a "serious house on
a serious earth." The "it" therefore refers to the church. Note how the stanza
continues:

A serious house on a serious earth it
is,
In whose blent air all our compulsions meet,
Are recognised, and
robed as destinies.
And that much can never be obsolete,
Since
someone will forever be surprising
A hunger in himself to be more
serious,
And gravitating with it to this
ground...

Even though the speaker in this poem
dares to mock the church, his actions give him away, as he takes off his bicycle clips
with "awkward reverence." Larkin argues that there is something intrinsically important
in churches, because they meet a felt need that we have, and when we find that we have
"a hunger in [ourselves] to be more serious," churches are the place that we will
gravitate to.