Dustin Lattimore, a Protestant, asked the question on Facebook. Earlier today, Dave Armstrong also also blogged about it. Mr. Armstrong, in denying the possibility of ex cathedra heresy, tried to be thorough but still hedged.
To get really philosophically heavy [Dear God.], at this point I would deny (in faith) that it could happen, but I don’t think it is an “impossible counterfactual” or “logically impossible” in all possible or conceivable worlds. [No, not all of them.] It’s the difference between philosophy and faith. In faith, I don’t believe it ever would or could happen in fact.
But it could in other worlds? Really?
Now, as I see it, this is like asking, “But what if God had made a round square?” There are no round squares. Round squares are logically impossible. And there is no other world in which the logically impossible exists. I don’t like to get into quantum apologetics. God can not make a round square any more than he can make a feline dog. God can’t make dark light; God can’t make a straight angle. Nor can he make a rock too big for himself to lift.
God does not give us a Church, if he gives us a Church at all (cf. Matt. 16:18), that can teach error. End stop. Let us consult Sacred Scripture on this point.
John 16:13. When the Spirit of truth comes, he will guide you into all the truth; for he will not speak on his own authority, but whatever he hears he will speak, and he will declare to you the things that are to come.
1 Tim. 3:15. If I am delayed, you may know how one ought to behave in the household of God, which is the church of the living God, the pillar and bulwark of the truth.
God does not make a round square; he does not make a bulwark of truth that teaches falsehood. Christ builds the Church on Peter; he does not give us a Church that could err.
The Church too tells us that a pope can not—no, not ever—teach error ex cathedra. Here is Vatican I.
We teach and define as a divinely revealed dogma that when the Roman Pontiff speaks ex cathedra, that is, when, in the exercise of his office as shepherd and teacher of all Christians, in virtue of his supreme apostolic authority, he defines a doctrine concerning faith or morals to be held by the whole Church, he possesses, by the divine assistance promised to him in blessed Peter, that infallibility which the divine Redeemer willed his Church to enjoy in defining doctrine concerning faith or morals.
God does not make a round square; God does not make a fallibly infallible pope.
But let us look further, to the wery Catechism of the Church.
889. In order to preserve the Church in the purity of the faith handed on by the apostles, Christ who is the Truth willed to confer on her a share in his own infallibility. By a “supernatural sense of faith” the People of God, under the guidance of the Church’s living Magisterium, “unfailingly adheres to this faith.”
890. The mission of the Magisterium is linked to the definitive nature of the covenant established by God with his people in Christ. It is this Magisterium’s task to preserve God’s people from deviations and defections and to guarantee them the objective possibility of professing the true faith without error. [God does not make a round square; God does not give an uncertain guarantee.]. Thus, the pastoral duty of the Magisterium is aimed at seeing to it that the People of God abides in the truth that liberates. To fulfill this service, Christ endowed the Church’s shepherds with the charism of infallibility in matters of faith and morals. [A pope can not teach error because he is protected from it by God.] The exercise of this charism takes several forms:
891. “The Roman Pontiff, head of the college of bishops, enjoys this infallibility in virtue of his office, when, as supreme pastor and teacher of all the faithful—who confirms his brethren in the faith he proclaims by a definitive act a doctrine pertaining to faith or morals. … The infallibility promised to the Church is also present in the body of bishops when, together with Peter’s successor, they exercise the supreme Magisterium,” above all in an Ecumenical Council. When the Church through its supreme Magisterium proposes a doctrine “for belief as being divinely revealed,” and as the teaching of Christ, the definitions “must be adhered to with the obedience of faith.” This infallibility extends as far as the deposit of divine Revelation itself.
892. Divine assistance is also given to the successors of the apostles, teaching in communion with the successor of Peter, and, in a particular way, to the bishop of Rome, pastor of the whole Church, when, without arriving at an infallible definition and without pronouncing in a “definitive manner,” they propose in the exercise of the ordinary Magisterium a teaching that leads to better understanding of Revelation in matters of faith and morals. To this ordinary teaching the faithful “are to adhere to it with religious assent” which, though distinct from the assent of faith, is nonetheless an extension of it.
God gives us an infallible teaching Church. That is a guarantee. There is no “but what if?” And not only does the pope have infallibility ex cathedra: Church councils have it; the bishops who teach in union with the Church have it.
It is just not possible for a pope to teach error. It has not happened; it can’t. End stop. It would mean that the Holy Spirit does not guide the Church into all truth; and that would make Jesus a liar when he speaks to the apostles in John 16:13.
If someone does think a pope might teach error ex cathedra, then it is pointless to start speculating about hypothetical other worlds in which God can make a round square or a bulwark of truth that errs. Direct me to a pope who has taught error ex cathedra. We’ve had 266 of them; there’s 2000 years of data. Search. Take all the time you need. Get back to us when you find one.