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."[1]
This has become an important precedent in ending discrimination against religious groups in public facilities such as libraries and after hours in schools.