The speaker describes a small survival trick: never get attached to a fallen flag, meaning a cause, ideal, or loyalty that has already collapsed. The admission that this is "ignoble" feels honest. There's a little shame in it, as if refusing to mourn is a kind of cowardice. But the speaker accepts the cost, because it lets them "live in the world."
The last three lines list what's left: a "God stick," a "heart of private night," and an "old rupture." The images are odd and a bit hard to pin down. A God stick might be something to lean on, like a walking staff, or a stripped-down faith. The private night suggests an inner life kept hidden. The rupture hints at an old break or wound that hasn't healed. The title, "Quietism," fits the mood: withdrawn, wary, and calm on the surface, with some hurt underneath. The tone is dry and guarded, more resigned than bitter.