“Freedom to manifest one’s religion or beliefs may be subject only to such limitations as are prescribed by law and are necessary to protect public safety, order, health, or morals or the fundamental rights and freedoms of others. . . . [P]risoners continue to enjoy their rights to manifest their religion or belief to the fullest extent compatible with the specific nature of the constraint.”

International Covenant on Civil and Political Rights, as ratified by the United States in 1992