Once I am sure there's nothing going on I step inside, letting the door thud shut.
Right away, you find out that the poem has a first-person speaker. The guy (at least we're assuming that he's a guy, for now anyway) enters the church only after making sure that "there's nothing going on," which suggests to us that he's not very comfortable being there during mass, a pancake breakfast, or any other churchy activities.
Once inside, he lets the door "thud shut" behind him. "Thud" is a great onomatopoeia, and seems to suggest a dull, lifeless sound.
Know what else? "Thud shut" is what they call in the poetry biz a spondee. Larkin's use of the spondee really helps us hear the sound of that door closing behind the guy, contributing to the creepy atmosphere of being alone inside a huge building.
Another church; matting, seats, and stone, And little books; sprawlings of flowers, cut For Sunday, brownish now; some brass and stuff Up at the holy end; the small neat organ;
When the speaker refers to the building as just "another church," he shows that he thinks that all are the same, since they all contain the same stuff, like "matting, seats, and stone/ And little books" (3-4).
Referring to the groups of flowers as random "sprawlings" also robs the church of the sense of orderliness and discipline that it's supposed to reflect. At the end of line 4, Larkin hits you with a really hard enjambment with the word "cut" left hanging off the end of the line. In one sense, this refers to the flowers, which have obviously been cut for presenting. The use of "cut," though, as opposed to "picked" or "plucked"—followed up with a description of the flowers as "brownish," or wilting—introduces some sense of harshness, and decay, into the poem.
In general, the speaker continues this trend of carelessly glancing at the sacred objects of the church by referring to them as "some brass and stuff" (5) and calling the altar "the holy end" (6) of the building. Basically, the guy is both ignorant of the "stuff" he's looking at and indifferent to it. Nothing here seems to strike him as meaningful, although he knows enough to realize that it's supposed to be.
And a tense, musty, unignorable silence, Brewed God knows how long. Hatless, I take off My cycle-clips in awkward reverence,
Line 7 brings our attention back to the vastness and silence of the church, which the speaker describes as musty. This could mean that he finds the church to be an outdated and boring thing, like a dusty book on a shelf.
But when the speaker says that this same silence is "unignorable," you really start to get a sense of the conflicting emotions Larkin explores throughout this poem. The speaker is bored, yet can't ignore the silence that looms over the church he's standing in.
The phrase "Brewed God know how long" refers to this silence (it's the silence that has been "brewed," not God—in case you're wondering), and suggests that the speaker has no clue how long it has taken for the church to develop the aura it has. He does know, though, that this aura is definitely there.
On a humorous note, the speaker's use of the phrase "God knows how long" also shows us that, even though the guy might not believe in God, the language of religion has gained such common usage that he can't help but use an expression like "God knows how long."
The final two lines of this stanza continue in this humorous tone, as the speaker, who isn't wearing a hat, wants to show his respect by taking off a piece of clothing. So he takes off his "cycle-clips," which are accessories worn to keep you pants from getting stuck in a bicycle chain. Handy! This gesture on the speaker's part could show his growing belief in the importance of religion, or it could actually be satirizing religion by showing how empty and ridiculous a custom like taking off your hat in church actually is.
When we reach the end of stanza 1, we discover that Larkin's poem follows a strict meter. What might that be? Well, technically speaking it's in iambic pentameter, with a rhyme scheme of ABABCADCD. For more on all that stuff, click on over to "Form and Meter." For our purposes here, just note that, in a very clever way, Larkin places the speaker's conversational tone in a tightly ordered poetic structure, which actually parallels the way that the speaker's casual ignorance toward religion plays out inside the tightly ordered structure of the church. And yes, folks. Larkin did mean to do that. Any minds blown yet?