Taken from Newdow vs. Congress (9th Circuit No. 00-16423)
"The Establishment Clause prohibits government from making adherence to a religion relevant in any way to a persons standing in the political community. Government can run afoul of that prohibition in two principal ways. . . . . The second and more direct infringement is government endorsement or disapproval of religion. Endorsement sends a message to nonadherents that they are outsiders, not full members of the political community, and an accompanying message to adherents that they are insiders, favored members of the political community."
Quoting from the concurring opinion of Justice O'Connor for Lynch v. Donnelly, 465 U.S. 668 (March 5, 1984) at 687-88.