Although it has attempted to create standards to differentiate religious beliefs and actions from similar nonreligious beliefs, the Supreme Court has never articulated a formal definition for religion. Given the diversity of Americans' religious experience since the Constitution was created, a single comprehensive definition has proved elusive.

In 1890, the Supreme Court in Davis v. Beason expressed religion in traditional theistic terms: “[T]he term ‘religion’ has reference to one’s views of his relations to his Creator, and to the obligations they impose of reverence for his being and character, and of obedience to his will.”

In the 1960s, the Court expanded its view of religion. In its 1961 decision Torcaso v. Watkins, the Court stated that the establishment clause prevents government from aiding “those religions based on a belief in the existence of God as against those religions founded on different beliefs.” In a footnote the Court clarified that this principle extended to “religions in this country which do not teach what would generally be considered a belief in the existence of God … Buddhism, Taoism, Ethical Culture, Secular Humanism and others.”

In its 1965 ruling United States v. Seeger, the Court sought to resolve disagreement between federal circuit courts over interpretation of the Universal Military Training and Service Act of 1948. The case involved denial of conscientious objector status to individuals who based their objections to war on sources other than a supreme being, as specifically required by the statute. The Court interpreted the statute as questioning “[w]hether a given belief that is sincere and meaningful occupies a place in the life of its possessor parallel to that filled by the orthodox belief in God of one who clearly qualifies for the exemption. Where such beliefs have parallel positions in the lives of their respective holders we cannot say that one is ‘in relation to a Supreme Being’ and the other is not.”

Welsh v. United States represented another conscientious-objector case under the same statute. The Court in this 1970 decision went one step further and essentially merged religion with deeply and sincerely held moral and ethical beliefs. The Court suggested individuals could be denied exemption only if “those beliefs are not deeply held and those whose objection to war does not rest at all upon moral, ethical, or religious principle but instead rests solely upon consideration of policy, pragmatism, or expediency.”

Following the expansive view of religion expressed in Seeger and Welsh, the Court in its 1972 ruling involving the Amish and compulsory school attendance suggested a shift back, to a more exclusive definition. The majority opinion in Wisconsin v. Yoder indicated that the free-exercise clause applied only to “a ‘religious’ belief or practice,” and “the very concept of ordered liberty precludes allowing every person to make his own standards on matters of conduct in which society as a whole has important interests.”

The Court in its 1981 decision Thomas v. Review Board further expressed its reluctance to protect philosophical values. The Indiana Supreme Court had ruled that a decision by a Jehovah's Witness to quit his job after he was transferred to a weapons-making facility was a “personal philosophical choice rather than a religious choice” and did not “rise to the level of a first amendment claim.” In overturning the Indiana decision, Chief Justice Warren Burger cautiously stated, “[o]nly beliefs rooted in religion are given special protection to the exercise of religion.” The Court found the worker's actions to be motivated by his religious beliefs.

Few have been satisfied by the Court’s attempts to define religion. Many of the Court’s definitions use the word “religion” to describe religion itself. In other cases, the Court’s explanations seem to provide little useful guidance.