The existence from the beginning of the Nation's life of a practice, such as tax exemptions for religious organizations, is not conclusive of its constitutionality. But such practice is a fact of considerable import in the interpretation of abstract constitutional language. . . .This Court's interpretation of the clause, accordingly, is appropriately influenced by the reading it has received in the practices of the Nation. . . .The more longstanding and widely accepted a practice, the greater its impact upon constitutional interpretation. History is particularly compelling in the present case because of the undeviating acceptance given religious tax exemptions from our earliest days as a Nation. Rarely if ever has this Court considered the constitutionality of a practice for which the historical support is so overwhelming.