Police Dept. of City of Chicago v. Mosley
In Police Dept. of City of Chicago v. Mosley, 408 U.S. 92, 96 (1972), the U.S. Supreme Court held that "government may not grant the use of a forum to people whose views it finds acceptable, but deny use to those wishing to express less favored or more controversial views."
This has become an important precedent in ending discrimination against religious groups in public facilities such as libraries and after hours in schools.