The van crashed about 9:45 a.m. near Vandalia, about 70 miles east of St. Louis, while en route home to Maryland from California, the St. Louis Post-Dispatch reported. Nine of the 11 occupants were thrown from the 15-passenger vehicle.
The names of the victims and their church had not been released.
Fayette County Coroner Bruce Bowen called the accident "my worst nightmare."
County Sheriff's Deputy Shawn Carter said the eastbound 2002 Dodge van ran into a median on Interstate 70, crossed the eastbound lanes and rolled several times.
Two survivors were hospitalized in serious condition. The condition of the others was not known.
St. Louis radio station KMOX reported the accident caused Illinois State Police to shut down the eastbound lanes of I-70 for a time, but the highway has since reopened.