Official Stamp

[O]ne of the greatest dangers to the freedom of the individual to worship in his own way lay in the Government's placing its official stamp of approval upon one particular kind of prayer or one particular form of religious services.

-- Supreme Court Justice Hugo Black, writing for the majority in Engel v Vitale (1962), ruling that New York State's requirement that all students recite the "Regent's Prayer" each day violated the establishment clause of the First Amendment (h/t vox.com)