“The Establishment Clause, unlike the Free Exercise Clause, does not depend upon any showing of direct governmental compulsion and is violated by the enactment of laws which establish an official religion whether those laws operate directly to coerce nonobserving individuals or not. This is not to say, of course, that laws officially prescribing a particular form of religious worship do not involve coercion of such individuals. When the power, prestige and financial support of government is placed behind a particular religious belief, the indirect coercive pressure upon religious minorities to conform to the prevailing officially approved religion is plain. But the purposes underlying the Establishment Clause go much further than that. Its first and most immediate purpose rested on the belief that a union of government and religion tends to destroy government and to degrade religion. The history of governmentally established religion, both in England and in this country, showed that whenever government had allied itself with one particular form of religion, the inevitable result had been that it had incurred the hatred, disrespect and even contempt of those who held contrary beliefs. That same history showed that many people had lost their respect for any religion that had relied upon the support of government to spread its faith. The Establishment Clause thus stands as an expression of principle on the part of the Founders of our Constitution that religion is too personal, too sacred, too holy, to permit its "unhallowed perversion" by a civil magistrate. Another purpose of the Establishment Clause rested upon an awareness of the historical fact that governmentally established religions and religious persecutions go hand in hand. The Founders knew that only a few years after the Book of Common Prayer became the only accepted form of religious services in the established Church of England, an Act of Uniformity was passed to compel all Englishmen to attend those services and to make it a criminal offense to conduct or attend religious gatherings of any other kind-- a law which was consistently flouted by dissenting religious groups in England and which contributed to widespread persecutions of people like John Bunyan who persisted in holding "unlawful [religious] meetings... to the great disturbance and distraction of the good subjects of this kingdom...."”

And they knew that similar persecutions had received the sanction of law in several of the colonies in this country soon after the establishment of official religions in those colonies. It was in large part to get completely away from this sort of systematic religious persecution that the Founders brought into being our Nation, our Constitution, and our Bill of Rights with its prohibition against any governmental establishment of religion.
Writing for the court, Engel v. Vitale, 370 U.S. 421 (1962).