Almost the only exception to the clergy-penitent privilege is for child abuse. Every state requires professionals, often including clergy, to report evidence of child abuse. But the requirements are aimed at abuse by a parent or guardian; they do not appear to cover the murder of a child by a stranger.

Almost as shocking as Pedro Hernandez’s claim to have murdered Etan Patz, the 6-year-old New York boy who disappeared while walking home alone in 1979, are reports that he admitted the murder three decades ago during a group confession of charismatic Christians at a Catholic church in Camden, N.J.

Asked why no one ever reported the confession, the prayer leader at the meeting said he didn’t think he could tell the police because Mr. Hernandez hadn’t confessed to him “one on one.” That response is almost completely backward as a legal matter.

Had Mr. Hernandez confessed “one on one” to a priest at the church, his lawyers almost certainly could have prevented prosecutors from using the confession at trial, thanks to an old rule known as the clergy-penitent privilege.

As early as 1827, English reformer Jeremy Bentham (who was more often a critic of religion than a defender) warned that compelling priests to tell what they learned through confession “would be an order to violate what by them is numbered amongst the most sacred of religious duties.”

By the 1920s, roughly 30 U.S. states had adopted a clergy privilege, often showing as much or more solicitude for the penitent as for the priest or pastor. “Unless the person confessing or confiding waives the privilege,” as the New York provision puts it, “a clergyman, or other minister of any religion or duly accredited Christian Science practitioner, shall not be allowed to disclose a confession or confidence made to him in his professional character as spiritual adviser.”

The privilege comes with the very steep cost that some wrongdoers may go free if their confession can’t be used. But a wrongdoer may be more willing to confess if confessions are protected, in which case the spiritual adviser or the penitent’s own conscience may encourage him to face up to his wrongdoing voluntarily.

[Editor’s note: This article is incomplete. The source for this document was originally published on online.wsj.com– however, the original URL is no longer available.]