In the history of the Church, whenever some grave heretical error has threatened the Faith and the path to eternal life, the faithful were saved by the surety of the teaching of the Apostolic See, that is, of the Roman Pontiff, and the teaching of Ecumenical Councils. One of the worst heresies, Arianism, was so popular that it was not defeated until TWO Ecumenical Councils each condemned it! But that is how heresies are recognized and condemned, by Pope and by Ecumenical Councils approved by Popes.
And now Catholics are saying that Vatican II and Pope Francis taught “manifest heresy”. “It’s just obvious that we’re right, and the Council and Pope are wrong.” That is the worst possible theological argument.
The term manifest heresy, as it is properly used, does not mean that the guilt of the accused needs no case or proof. It means that the heresy is not hidden in the heart and mind, nor even kept to expressions in private to few persons. Rather, the manifest heresy is a public heresy. The word manifest does not imply that the error itself is obviously contrary to dogma.
Many heretics, perhaps most, believe themselves to be adhering to either the correct understanding of the Catholic Faith, or at least to truth. It is less common for a heretic to state that they know what the dogma of the Church is, and they reject that dogma. Rather, the heretic would call the teaching of the Church erroneous, and would ordinarily deny that it is infallibly taught.
Take the case of Honorius as an example. He has had faithful theologians and priests defending him against the charge of heresy for many centuries now. Pope Saint Agatho, Saint Robert Bellarmine, Saint Maximus the confessor, and Pope John IV all defended him, as did Cardinal Manning, a father of Vatican II. And it seems clear that Vatican I took its infallible teaching on the freedom from blemish of the Apostolic See from the Letter of Agatho defending Pope Honorius. So while some Catholics today adamantly accuse Honorius of heresy (so that they obtain a precedent for accusing Francis), other Catholics still defend Honorius. Even in that famous case, it is not “manifest” in the sense of obvious, that Honorius is guilty.
The Sixth Council tried to convict Honorius of heresy, but Pope Saint Leo II changed the charge from heresy to negligence.
As for Vatican II, it is the role of Ecumenical Councils, approved by the Pope, to distinguish heresy from dogma. They are only able to do this by the guarantee of the Holy Spirit that the Council will not err gravely on doctrine or discipline, and that the definitive teachings of the Council are infallible. No Ecumenical Council approved by the Pope has ever been known to have taught heresy. And the clamoring on the internet against Vatican II is not proof to the contrary. Rather, no Council has convicted a prior Council of heresy, and each Pope, according to Vatican I, has the charism of never failing faith. Heresy being a grave failure of faith, no Pope can teach or commit heresy.
To accuse Vatican II or any other approved Council or any Pope of heresy is to abandon the Catholic faith, which establishes its dogmas, and condemns the contrary heresies, via Popes and Councils.