The Free Exercise Clause at the very least was designed to guarantee freedom of conscience by prohibiting any degree of compulsion in matters of belief. It was offended by a burden on one's religion. The Establishment Clause can be understood as designed in part to ensure that the advancement of religion comes only from the voluntary efforts of its proponents and not from support by the state. Religious groups are to prosper or perish on the intrinsic merit and attraction of their beliefs and practices.
Harry A. BlackmunThis Court repeatedly has recognized that the whole subject of the domestic relations of husband and wife belongs to the laws of the States and not to the laws of the United States.
Harry A. BlackmunThe mixing of government and religion can be a threat to free government, even if no one is forced to participate.... When the government puts its imprimatur on a particular religion, it conveys a message of exclusion to all those who do not adhere to the favored beliefs. A government cannot be premised on the belief that all persons are created equal when it asserts that God prefers some.
Harry A. Blackmun