Yet many people know exactly where to find it, and some are convinced that it has to go. On two occasions since it opened in June, nearby residents, to humiliate worshipers, have left the bloodied heads of pigs outside the mosque’s door. Shortly thereafter, regional authorities resumed their push to shut it down, after nearly 15 years of trying — and failing — to prevent it from opening.

This modest, nondescript house of worship, the largest yet to open in the region, has become a symbol of the precarious position occupied by French Muslims, the country’s largest minority, in a society reeling from terrorist violence and hurtling toward a watershed presidential election. If the mosque is forced to close, it would violate the age-old promise of a proudly secular republic never to discriminate among citizens on the basis of race or religion.