There is an agreement between the priest and the layman that the conversation is only with them and God.
What's the justification for others to honour this agreement? Generally, when there's an agreement between two people to conceal a crime, the law doesn't care (or may consider the person concealing the crime guilty of a crime themselves).
The only thing similar is the lawyer client agreement. Even if the lawyer knows you committed the crime, he can be removed from the bar if he doesn't defend you properly.
A few things:
- there are other similar arrangements. Doctor-patient privilege, for instance.
- you're confusing different concepts. The defense lawyer's duty to provide a proper defense is different from his or her duty of confidentiality.
- even lawyer-client confidentiality isn't absolute. There are circumstances when a lawyer is allowed to breach the confidentiality of a client... generally to prevent a severe violent crime.
If the confessional agreement is broken people simply won't confess. They only go to try and get absolution.
Some people, maybe. Most? Probably not. The police and the courts don't care about most sins.
Keep in mind that we're talking specifically about mandatory reporting of child abuse. I don't see how this would affect the reluctance of anyone to confess besides people who might mention child abuse in the course of their confession.
The priest can not grant absolution unless they come clean. Meaning admit what they did and stop doing it and ask forgiveness from the victim.
Yes: to a certain extent, it would compromise how confession works... just as mandatory reporting by doctors means that some patients might hold information back from their physicians... but is this cost so high that it justifies foregoing the benefits that would come with mandatory reporting?
In the case of physicians, we as a society say that the societal benefits of the law outweigh the cost. What's so different in the case of priests?
If a person goes to a priest they are obviously feeling guilt the priest may actually push them to come clean. Doing away with the confessional they may continue to do it.
In the article I linked to, the priest found out about the abuse during the confession of the victim. He also - according to reports - told the girl to "sweep it under the rug" and not to report the abuse.