The Lemon Test
In Lemon v. Kurtzman (1971), the U.S. Supreme Court established a three-pronged test to determine whether a law violates the First Amendment to the U.S. Constitution, which bars any law "respecting an establishment of religion."
1. The law should fulfill a secular public purpose.
2. The law should not have the primary effect of advancing religion.
3. The law should not lead to "excessive entanglement" of church and state.
In Agostini v. Felton (1997), the Court provided guidelines for aid to parochial schools. Such aid is permissible, said Justice Sandra Day O'Connor, writing for the majority, if it is "allocated on the basis of neutral, secular criteria that neither favor nor disfavor religion, and is made available to both religious and secular beneficiaries on a non-discriminatory basis."