"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."