Would not embracing Vatican II with it's teachings on religious liberty constitute heresy?
Did he embrace them? Secondly, no, religious liberty as a term is not heretical. 1) it's never been defined with the note of
de fide and 2) there's probably a different understanding of the phrase depending on who you ask. I've seen a lot of people (conservative Novus Ordites) asser that it basically means religious toleration.
I think we just had a round of where I was defending you against the charge of heresy from RomanTheo. When we asser that someone is a heretic, we'd better have 1) chapter and verse of which dogma they're directly denying and 2) proof that the person denied it.
To charge Siri with heresy, I'd need to see 1) citations from Siri himself and 2) an explanation for why it directly contradicts something that's
de fide. And then if you confronted him about it, would he recant?
It's not really relevant to the discussion here, since if Siri materially held the office, then he held the office.