Bonta gives us a tiny scene: a place where people go to church, and leaves changing color around it. There's no story here, just a moment of looking, and the feeling that comes with it.
The second stanza is where the poem turns. A mordant is a substance that fixes dye so a color stays in fabric. The speaker says the north works like that for "our wine-laden loss." The word "wine" suggests the deep reds and purples of autumn leaves, but also the wine of communion, which ties the church to the season. Loss seems to be what the cold, northern landscape sets in place, making it vivid and lasting rather than letting it fade.
The mood is hushed and reflective, a little sad but also rich. The speaker doesn't spell out what was lost. The poem suggests that grief, like autumn, can be strangely beautiful. The word "our" makes it shared, something a whole community feels together.