This question already has an answer here:
Unfortunately, we can hear some lay Catholics (sometimes even a bishop) say that: "We can have reasonable hope that all will be saved". It seems obvious that the statement is untrue, however, is it a heretical statement? And would for example bishop who persists in this error becomes a formal heretic?
We know that the Church condemned the following proposition:
If anyone says or thinks that the punishment of demons and of impious men is only temporary, and will one day have an end, and that a restoration (ἀποκατάστασις) will take place of demons and of impious men, let him be anathema.
It seems that corollary of this statement is:
Corollary: Punishments of impious men will never end.
Imagine now that the statement: "We can have reasonable hope that all will be saved" is true. However, if the hope were to be reasonable, then the object of hope should be really possible, namely, it should be really possible that all are saved. But if all were saved, then the previous corollary would really be attributing some property to an empty set and then the condemnation would be vacuously true. But the opposite of the corollary would also be true (because we can attribute anything to an empty set, see a previous link to "Vacuous truth") in that case. Therefore, the case where all are saved seems logically inconsistent if we want to say that condemnation can not be empty.
Edit. My question is not duplicate. I am not searching an answer to a viewpoint on universal salvation, but I explicitly deny that as untrue. I am searching for what kind (degree) of error is to hold universal salvation (or rather, that "we can have reasonable hope that all will be saved").