If the Pope said that 2 + 2 = 5, would you believe him?

"No, of course not!" you would say, "The axioms of arithmetic are absolutely immutable.  No one, the Pope included, could ever change that."  If you agree with this (and, you must), then the axioms of Catholic revelation are just as immutable, for the axioms of arithmetic and those of Catholic revelation both come from the one and same source, the immutable One and Triune God.  The First Vatican Council recognized this fact stating,

Pope Pius IX, Vatican Council, Session 3, Chapter 4, #14, ex cathedra: "Hence, too, that meaning of the sacred dogmas is ever to be maintained which has once been declared by Holy Mother Church, and there must never be any abandonment of this sense under the pretext or in the name of a more profound understanding."

Pope Pius IX, Vatican Council, Session 3, Canon 4, ex cathedra: "If anyone says that it is possible that at some time, given the advancement of knowledge, a sense may be assigned to the dogmas propounded by the church which is different from that which the church has understood and understands: let him be anathema."

Of course, some neo-conservatives claim that only the present Magisterium can interpret what the infallible Magisterium has solemnly proclaimed.  This view is, of course, manifestly absurd, for if we need the present Magisterium to interpret the "past" Magisterium who, then, is going to interpret the present Magisterium, and who is going to interpret those who interpret the present Magisterium?  And, who will interpret them, and so forth?  The answer to this dilemma is, of course, that the very words of the solemn definitions given by the Supreme Magisterium are their own interpreter.

Which is of higher authority, the US President or the US Constitution?  Of course, the answer is the latter.  The US Constitution is, however, just a bunch of organized letters on pieces of paper/parchment.  One cannot call the "US Constitution" up on the telephone, send it a letter or email, or stop by and visit it to see what it "thinks" about a particular issue.  Yet, the Constitution is its own interpreter, for its very own words make it clear what is and is not constitutional. This is why US Supreme Court Justices issue opinions about the Law, because they recognize that US Constitutional Law has its own "immutable" aspects to it.

While the US Constitution is based upon the deistic principles of the Enlightenment, Catholic revelation comes from the Creator of the Universe, the author and giver of both divine and natural law.  Unlike the US Constitution, it is absolutely immutable, but like the US Constitution, the very words of the solemn Magisterial declarations are self-evident, and we need no one to "interpret" those words for us.  The words are, themselves, their own interpreter and are guaranteed to be absolutely true by the power of the Holy Spirit, the Third Person of the Blessed Trinity, God Himself.

Galileo Galilei

It was once the theological teaching of the Church that the Earth was at the physical center of the Universe:


"Therefore . . . , invoking the most holy name of our Lord Jesus Christ and of His Most Glorious Mother Mary, We pronounce this Our final sentence: We pronounce, judge, and declare, that you, the said Galileo . . . have rendered yourself vehemently suspected by this Holy Office of heresy, that is, of having believed and held the doctrine (which is false and contrary to the Holy and Divine Scriptures) that the sun is the center of the world, and that it does not move from east to west, and that the earth does move, and is not the center of the world; also, that an opinion can be held and supported as probable, after it has been declared and finally decreed contrary to the Holy Scripture, and, consequently, that you have incurred all the censures and penalties enjoined and promulgated in the sacred canons and other general and particular constituents against delinquents of this description. From which it is Our pleasure that you be absolved, provided that with a sincere heart and unfeigned faith, in Our presence, you abjure, curse, and detest, the said error and heresies, and every other error and heresy contrary to the Catholic and Apostolic Church of Rome."

Yep, the Church can err; not the Holy Spirit, of course, but human beings.  In the case of Galileo, it was not the first time, nor will it be the last.  The Magisterium is infallible just as the One and Triune God is immutable, but unlike the latter, it is just not infallible all the time.  If such were the case, then the Pope would be perfect, which would mean that he was not a human being, but the Incarnate God, Jesus Christ. The Pope's imperfection is just proof that he is a fallible human being.  His charisma of infallibility stems not from him, but from the immutable God, the Holy Spirit.  Because he is human, he is incapable of exercising that charisma all the time in everything that he does, but only under special circumstances and only when he so indicates.

The verdict against Galileo did not meet the criteria for Papal infallibility.  Some, Catholic and non-Catholic, are scandalized by the fact that the geometry of the solar system and Universe (the latter, of course, came well after the life of Galileo) were not part of the Deposit of Faith.  Of course, neither was the recipe for a Big Mac, the formula for Nexxus hair conditioner, or the thrust-to-weight ratio for a working space shuttle given as part of divine revelation either.  Were such truths necessary "for us men, and for our salvation"?  I do not believe so, which is probably why the One and Triune God did not include such facts as part of His general revelation to the World.