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The decidedly Christian nature of these prayers must not be dismissed as the relic of a time when our Nation was less pluralistic than it is today. Congress continues to permit its appointed and visiting chaplains to express themselves in a religious idiom. … To hold that invocations must be nonsectarian would force the legislatures … and the courts … to act as … censors of religious speech. … Government may not mandate a civic religion that stifles any but the most generic reference to the sacred any more than it may prescribe a religious orthodoxy …