“Fifty years have passed since the words ‘under God’ were added, a span of time that is not inconsiderable given the relative youth of our Nation. In that time, the Pledge has become, alongside the singing of the Star-Spangled Banner, our most routine ceremonial act of patriotism; countless schoolchildren recite it daily, and their religious heterogeneity reflects that of the Nation as a whole. As a result, the Pledge and the context in which it is employed are familiar and nearly inseparable in the public mind.”