"When we cover Confession in Wednesday Night Sunday School, a child will always ask if a priest can tell the cops if someone confesses to a murder. I say of course not; and that leads to the idea that the priest forgets your sins. And then someone will ask: but what if he doesn't forget?
To answer this I first refer to these gracious verses:
Is 43:25 "I am He who blots out your transgressions for my own sake, and I will not remember your sins."
Jer 31:34 "I will forgive their iniquity, and I will remember their sin no more."
Heb 8:12 "For I will be merciful toward their iniquities, and I will remember their sins no more."
(They're highlighted in my Bible, but I'll just paraphrase them if time is short.)
This does beg the question: how does an omniscient God not remember something? Well, isn't it in the nature of a loving parent to not remember? Those verses remind me of my father's memory of my several-thousand-or-so, umm, childhood misbehaviors:
"Hey remember that time that I...."
"Mmm...no...I don't think I remember that."
"Yeah, but it was like thus and so, and..."
"Uh-huh...I just don't much remember you being bad."
"That's probably just as well."
And it's the same with my kids...were they bad? I suppose...yeah, if I put my mind to it they were bad some. Well, not really bad; they were just kids, learning to be good...I don't remember that much about it. Not remembering your child's sins isn't a matter of deleting the data from your brain, making them irrecoverable; it just means the forgiving parent no longer pays any attention to them, and is not going to ever pay any attention to them.
And if God will remember our sins no more, then the priest, acting in persona Christi, would also not remember them, at least in a sacramental sense. That is, even if the priest through his human frailty does remember our sins, he may act only as though he does not remember them, in the same way that God does not remember them.
Thus the Seal of the Confessional."