Once I am sure there's nothing going onI step inside (1-2)
These opening lines reveal that the speaker isn't all that comfortable with entering the church while actual churchgoers are inside. He prefers to explore the place when it's empty. While he doesn't know much about church stuff, he knows enough to avoid the place when real churchgoers are there. He might be afraid of offending them by doing something wrong, or he might want to explore the church with a sense of total freedom. In this opening line, Larkin sets the stage for the entire poem, which will concern the theme of feeling out-of-place among a community of true believers.
Another church: matting, seats, and stone,And little books; sprawlings of flowers, cutFor Sunday, brownish now (3-5)
By referring to the building as "Another church," Larkin's speaker robs the church of its unique value as a holy place. In general, this first stanza really sets up an ironic distance between the speaker and the value that the church is supposed to have. The mention of "little books" and "sprawlings of flowers" that have gone brown reflect a certain pettiness and disarray to the insides of the church, which is only set up to look really nice for Sunday mass, and then left to sprawl and get dirty until the following mass. These images suggest a certain phoniness about the church, which is made to look good for the parishioners, but not treated in a holy way at all when no one's looking. You'd think that if the church were truly a holy place, someone would be taking care of it even when no one was looking.
Hatless, I take offMy cycle-clips in awkward reverence (8-9)
Here, the speaker takes off the cycle-clips that keep his trousers from getting caught in his bicycle chain. He does this because he doesn't have a hat and feels like he should take off some article of clothing to show his respect. These lines might show the speaker's genuine attempt to show respect for the church, but, at this early point in the poem, the gesture seems like Larkin's sarcastic way of saying that church customs are superficial and meaningless—more a matter of habit than actual belief.