The speaker walks through an abbey, a place built for worship, and thinks of a religion as "innocent as the grass": simple, unforced, part of the natural world. It's a lovely idea, but the poem doesn't let it stay lovely for long. The speaker is in pain from new shoes, has heartburn, and ends up with a griping belly that makes him vomit and shit.
The contrast is the point. A calm, almost pure image of faith is dragged down by the plain, embarrassing demands of the body. The blunt, crude language in the last lines makes that drop feel sudden and a little comic. The title, "Man of faith," sounds a bit ironic against all this, though it may also suggest that belief has to live alongside discomfort and ordinary human mess. The mood is wry and unsentimental: the speaker seems to see his own situation with humor, even while he suffers.