The law says, "No government shall impose or implement a land use regulation in a manner that imposes a substantial burden on the religious exercise of a person, including a religious assembly or institution," unless it meets very strict conditions. Nor may any government inflict a regulation that discriminates "on the basis of religion or religious denomination."
Before this law, cities usually got away with such behavior, because zoning authority gives them broad discretion to hinder religious groups in the outwardly neutral guise of architectural preservation or traffic control. It wasn't hard to discriminate without being obvious enough to get in trouble.
In passing the measure, Congress sided with unpopular minority sects that often found themselves blocked at every turn by local governments. So even if the opposition to the Cordoba plan were cloaked as an impartial effort to preserve the character of the neighborhood, it would run smack up against this law.
This case, though, is even simpler. Most of the time, when local authorities throw up roadblocks before unwanted religious groups, they do so in a way designed to conceal their real motive. Here, though, the critics object to the center precisely because it involves Muslims doing nothing more than practicing their religion.
There is nothing covert or subtle about the opponents' motives. To stop the building on the grounds that Islam has no place near Ground Zero would clearly defy the law's ban on religious discrimination.
That prohibition is a credit largely to conservatives who understood the dangers of putting religion at the mercy of government. The law was an effort by Republicans (and many Democrats) to protect the rights of believers -- especially despised minorities.
The law recognized the importance of assuring the same freedom for them as for everyone else. That objective made sense 10 years ago, and it still does.