In the 1947 case of Illinois ex rel. McCollum v. Board of Education, Vashti McCollum, a resident and taxpayer in Champaign County, Illinois filed suit against the local school board for allowing religious instruction during school hours on public property. The program was voluntary but McCollum argued that it violated her son's First Amendment rights by effectively forcing him to participate in religious activities or face social ostracism. She also claimed that it violated the Establishment Clause which prohibits government from establishing an official religion or favoring one religion over another. The Supreme Court ruled 8-1 in favor of McCollum stating that this practice did indeed violate the Establishment Clause as well as infringe upon individual freedom of religion protected under the First Amendment. This landmark ruling set a precedent for future cases involving separation between church and state within public schools.

In the dissenting opinion for Illinois ex rel. McCollum v. Board of Education, Justice Reed argued that the religious instruction program did not violate the Establishment Clause because it was voluntary and parents had to give written consent for their children to participate. He maintained that no state coercion existed since students could choose secular activities during this time period if they opted out of religious education classes. Furthermore, he contended that public schools were merely providing a convenient location for these classes and were not endorsing or promoting any particular religion themselves; thus, there was no violation of separation between church and state principles as per his interpretation. The dissent also emphasized on respecting parental rights in deciding their children's religious upbringing without government interference.