Instead of describing a church service in the usual way, the poem gives us four quick, odd fragments, as if the speaker were glancing around a congregation and catching only pieces. The first is a "strange company," which suggests that the people gathered there feel unfamiliar, or that the speaker feels like an outsider among them. Then come harsher images: "a great store of gall" and "an impertinent sling." Gall can mean bitterness or nerve, and a sling is a weapon, so these lines hint at resentment, rudeness, or attack sitting beneath the polite surface of worship.
The last image, "a child in a well," is the most unsettling. It could point to innocence trapped or in danger, or to someone calling out from far below while everyone above ignores them. The poem never explains, and that's part of its effect.
The mood is uneasy and quietly critical. The speaker seems watchful and skeptical, noticing tension and vulnerability rather than comfort. The poem leaves the reader to decide what holds these images together.