U.S. District Court Magistrate Judge Donald J. Scheer said during his ruling in Detroit that federal prosecutors had provided the court with "very disturbing information."
Scheer said that he agreed with defense attorneys that possessing firearms, mistrusting the government and practicing military exercises are not illegal.
However, he said, "there comes a point when those activities ... may lead to actions which would constitute evidence of criminal offense."
Defense attorney James Thomas, who represents Josh Stone, 21, appealed Scheer's ruling and expected others to do the same. A hearing wasn't expected until next week.
Thomas said the charges are serious, but "the indictment is nothing more than a piece of paper."
The eight are suspected members of a group called the Hutaree, which is based in southern Michigan's Lenawee County. A ninth suspect is being held in Indiana.
All the defendants were charged this week with seditious conspiracy, or plotting to levy war against the U.S., attempting to use weapons of mass destruction and other crimes.
They planned to make a false 911 call, kill responding police officers and set off a bomb at the funeral to kill many more, according to prosecutors. The FBI said it broke up the plot with the help of an undercover agent and informants.
The Hutaree are self-proclaimed "Christian warriors" who trained themselves in paramilitary techniques in preparation for what they say on their Web site is a battle against the Antichrist.