Infallible. Binding.
Two different things.
Catholics are bound to avoid the use of artificial contraception. The teaching is binding.
The teaching has never been proclaimed to be infallible.
It is likely a good candidate to be proclaimed to be infallible, and may be infallible, already. But Pope Paul VI actually struck language from the text of Humanae Vitae declaring the prohibition to be infallible.
Certainly, it meets the attributes of the infallible teaching of the ordinary Magisterium. It is a teaching on faith and morals. It has been taught to the universal Church. It has been taught consistently since the first century.
Yet, Pope Paul held back. Some will say he lost his nerve. I try to avoid judging even dead popes. I will leave it to some other pontiff to teach what is what in this regard.
But whether the teaching was infallible or not, it is binding.
It was once taught authoritatively that it was immoral to take interest on loans. At that point, this teaching was binding on all Catholics.
That's no longer taught.
Was the teaching infallible? It had the attributes of a teaching of the ordinary Magisterium which was universally taught to the universal Church consistently for the entire history of the Church, up until when it was changed.
Whether it was infallible is problematic (I've read arguments both ways.). But in any event, it sure as heck was binding.
As are all teachings by popes on matters of faith and morals addressed to the universal Church.
Catholics don't get to pick and choose between teachings.
But a pope can also give opinions as a theologian, or as a private person, not addressing them to the universal Church as authoritative teaching on matters of faith and morals. And in these teachings, a pope can err.
The reason for all this is that we interpret Jesus' promise that the gates of Hell will not prevail against His Church to mean that the authoritative teaching office of the Catholic Church can not teach error, and thus, lead Christians into error. Like you, we think right doctrine is important, as well. Vitally important.
The pope will not lead the faithful off the cliff of heresy. This is the promise.
To say otherwise is to cease to believe in the promise of Jesus Christ to His Catholic Church.
Thus, to say that a pope could write and issue an encyclical addressed to the universal Church which would be heretical is extremely problematic. This is especially true since Catholics are bound even by teachings which are not made infallibly.
In any event, no one alive could unilaterally judge a pope to be in heresy. Theologians and bishops of sufficient rank could propose that such has happened, but none have the authority to definitively judge a pope thusly. A pope, himself, could make that judgement, and my memory dimly reminds me of Pope John XXII, I think, who held a private opinion which eventually turned out to be material heresy (because he held a private opinion that turned out to be heresy PRIOR to the Church's determination that it was, indeed, heretical, he couldn't be said to have been a formal heretic). He believed that the souls of the blessed departed do not see God until after the Last Judgment.
How did he know it was heresy? Well, he himself wasn't sure of the opinion, so he appointed a commission to study the question. They came back and said, "Sorry, boss, it's heresy." But EVEN AT THAT POINT, he was not yet in formal heresy. He was initially unconvinced that he was wrong. Though the commission could PROPOSE that he was wrong, it could not DEFINITIVELY JUDGE him to be wrong.
He also NEVER TAUGHT HIS OPINION AS CHURCH TEACHING. He explicitly stated that it was his own opinion, that he could be wrong, and that he in no way ever intended to teach that which was contrary to Scripture and Sacred Tradition.
He eventually changed his mind, and accepted that his former view had been wrong. And that is when the particular doctrine in question was definitively decided by and for the Church. When the pope decided it.
Also, to a limited degree, the judgements of a following pope can be made on a deceased pope. A successor pope can judge a predecessor, within certain limits. Leo II condemned the late Honorius for failure to defend the faith from heresy. But not for actually teaching heresy to the universal Church.
But no one on earth has the authority to definitively judge the current pope to be a heretic. Can't do it.
Insofar as some folks here do that, they are themselves guilty of material heresy.
Remember that for the most part, the charism of infallibility is a negative charism. It doesn't mean that the pope is going to wake up every day and preach what the Church needs. It means that the pope is protected from teaching falsely to the universal Church. He may teach incompletely. He may teach poorly. He may fail to teach what is true. But he will not teach what is false.
We have Christ's Word on it.
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