Why did School District of Abington Township v. Schempp prohibit officially required Bible reading in public schools?

In School District of Abington Township v. Schempp (1963), the U.S. Supreme Court ruled that officially mandated Bible reading and prayer in public schools violate the establishment clause of the First Amendment.1

The Court determined that the establishment clause requires the state to maintain neutrality, not only between different religious groups but also between religious believers and nonbelievers. To be constitutional, a government enactment must have a secular legislative purpose and a primary effect that neither advances nor inhibits religion.1

The Court found that Bible readings and prayers in schools were religious exercises that failed this test. It rejected the argument that these practices served a secular purpose of moral inspiration, noting that the government cannot use its authority to promote religion or allow a majority to use state machinery to practice its beliefs. Furthermore, the Court held that allowing students to be excused from these exercises did not cure the constitutional violation, as the state remains prohibited from promoting religious activity at the public's expense.1