The poem opens with a question that sounds like a challenge: who but the body could speak about sin? The speaker seems to suggest that sin isn't an abstract idea. It lives in flesh, in desire, in the things we do and feel, so only the body has the standing to talk about it.
The second and third stanzas turn to beauty. Each of us, the speaker says, takes on "a mistaken beauty." That could mean we wear an appearance that isn't quite true to us, or that the beauty we're admired for is somehow misread. The comparison that follows is the most striking part: it's like "the name of a god / in common letters." A sacred name is spelled with the same ordinary letters as any other word, and in the same way, something holy or lovely is housed in an ordinary human body.
The mood is quiet, thoughtful, and a little uneasy. The speaker doesn't scold or celebrate. The title, "Incommensurable," hints that the divine and the bodily can't be measured against each other, yet here they share one shape.