The mixing of government and religion can be a threat to free government, even if no one is forced to participate. When the government puts its imprimatur on a particular religion, it conveys a message of exclusion to all those who do not adhere to the favored beliefs. A government cannot be premised on the belief that all persons are created equal when it asserts that God prefers some.
—Harry Blackmun (1908 – 1999), Associate Justice of the Supreme Court of the United States from 1970 until 1994. The quote is from his 1992 concurring opinion in Lee v. Weisman, which said public school graduation ceremonies could not including officially sanctioned prayer.
Source: “Justice Blackmun’s legacy lives on in church-state, commercial-speech decisions,” by Tony Mauro, from firstamendmentcenter.org, published March 5, 1999, the day after Harry Blackmun’s death.