Slippery slopes are fallacies for a reason.

Slippery slope is a type of fallacy. It isn't fallacious always.

'in its barest bones, a slippery-slope argument is of the following form:

“If A, which some people want, is done or allowed, then B, which most people don’t want, will inevitably follow. Therefore, let’s not do or allow A.” 

The fallacy occurs when that form is not fleshed out by sufficient reasons to believe that B will inevitably follow from A'

(https://intellectualtakeout.org/2016/03/not-every-slippery-slope-argument-is-a-fallacy/)

Saying that this would create a precident to include other crimes being required to be reported is not fallacious.

If you subpeona a priest who saw someone commit a crime, all he has to say is "I cannot testify, it is against my religion."

That's just blatantly incorrect. They're not required to report on stuff they're told in confessionals and that's all. They're still required to report on crimes they witness, just like everyone else. Do you think lawyers are t required to report crimes they witness?

Do you understand the issue? The priest can't ever say "I can't testify because I heard it in confession" because that in and of itself is a breach of the seal of confession.

So he can only say "I cannot testify" and we all have to leave it at that.

Yes, just as a lawyer would have to do when questioned about a client. Anything they did outside of attorney-client privledge they must speak about, it'd be the same for the clergy. It's not an issue for lawyers, so I don't see an issue for the clergy.

In an ideal world they could hear the confessional and check up on the victim. I'm sure this won't always happen, but it may. If they're required to report it, they'll never be told, so can't act on it.

I don't like religion, and especially organized religion. However, this steps too far into a government that forcing it's way into people's lives that I don't like.