An objection, however, is often made to the doctrine of infallibility, in limine, which is too important not to be taken into consideration. It is urged that, as all religious knowledge rests on moral evidence, not on demonstration, our belief in the Church's infallibility must be of this character; but what can be more absurd than a probable infallibility, or a certainty resting on doubt?— I believe, because I am sure, and I am sure, because I supposed. Granting then that the gift of infallibility be adapted, when believed to unite all intellects in one common confession, the fact that it is given is as difficult of proof as the developments which is to prove, and nurgatory therefore, and in consequence improbable in a Divine Scheme. The advocates of Rome, it has been urged, "insist on the necessity of an infallible guide in religious matters, as an argument that such a guide has really been accorded. Now it is obvious to inquire how individuals are to know with certainty that Rome is infallible... how any ground can be such as to bring home to the mind infallibly that she is infallible; what conceivable proof amounts to more than a probability of the fact; and what advantage is an infallible guide, if those who are to be guided have, after all, no more than an opinion, as the Romanists call it, that she is infallible?" 81.1
This argument, however, excepted when used, as is intended in this passage, against such persons as would remove all imperfection in the proof of Religion, is certainly a fallacious one. For since, as all allow, the Apostles were infallible, it tells against their infallibility, or the infallibility of Scripture, as truly as against the infallibility of the Church; for no one will say that the Apostles were made infallible for nothing, yet we are only morally certain that they were infallible. Further, if we have but probable grounds for the Church's infallibility, we have but the like for the impossibility of certain things, the necessity of others, the truth, the certainty of others; and therefore the words infallibility, necessity, truth, and certainty ought all of them to be banished from the language. But why is it more inconsistent to speak of an uncertain infallibility than of a doubtful truth or a contingent necessity, phrase which present ideas clear and undeniable? In sooth we are playing with words when we use arguments of this sort. When we say that a person is infallible, we mean no more than that what he says is always true, always to be believed, always to be done. The term is resolvable into these phrases as its equivalents; either then the phrases are inadmissible, or the idea of infallibility must be allowed. A probable infallibility is faith and obedience towards a person founded on the probability of his never erring in his declarations or commands. What is inconsistent in this idea? Whatever then be the particular means of determining infallibility, the abstract objection may be put aside. 81.2 [John Henry Cardinal Newman, Essays on the Development of Christian Doctrine, 58-9]
Before there was Catholic Answers, there was John Henry Newman, the apostate from Anglicanism. On the issue of the infallibility of the Roman Church, there is surely some continuity between the two. Protestants, in response to Roman claims on the infallibility of the Church, rightly ask how one can be infallibly certain of that. Newman's response is to say that this same argument can be used against the infallibility of the Apostles and the Scripture, the former probably as a polemic against Anglicanism and the latter against Protestantism as a whole. Since the same argument can be used against other views, either one has to abandon all usage of the words "infallibility, necessity, truth and certainty," or the argument is to be discounted altogether.
Newman's move, while an interesting play on his part, does not actually solve the problem. One can use the same argument to argue for the infallibility of the apostles or the infallibility of Scripture, or basically any authority. Put that (X is infallible) as the thesis, then when one's opponents inquire how one knows that (X is infallible) is true, then claim that either the thesis X is infallible (e.g. Scripture is infallible) is true, or that these words (infallibility, necessity, truth, and certainty) have no meaning whatsoever. Once the form of the argument is recognized, we see that Newman's argument is begging the question. It works only to the extent that his thesis is seen to be the default by which all others are to be judged, but since the thesis itself is questioned, one cannot claim that as the default.
So Rome's claims of authority is circular, and in the end it is no more certain than other claims. That it is circular does not necessarily make it wrong, but it must be admitted that the reason why Romanism is right is because one bases one's faith on the claimed authority of the Roman church (i.e. Sola Ecclesia). The major problem for Romanism now is the inconsistency of its authority figure. Ever since Vatican II, the Roman church has been ever shifting in its positions on all manner of Christians doctrines. Where once heretics and schismatics were anathemized, now Protestants are regarded as "separated brethren" and Muslims and Jews part of the plan of God (Vatican II Document Lumen Gentium). If one's authority is ever shifting, how can it function as the axiom of one's system?
Against Laudian Anglicanism, it is impossible to see the Church Fathers as being authoritative for understanding the Christian faith, since one does not have the entire corpus of Patristic writings and there is always the possibility, and indeed there are, divergences between the various church fathers. As for apostles, the only way we know what the apostles have taught is in the Scriptures. Supposed apostolic tradition mediated to the church fathers always have the question how much is actual mediation, and how much is addition, and thus one cannot appeal to the church fathers as necessarily indicative of apostolic tradition.
Scripture alone can stand as its own authority. The canon of Scripture is fixed and it is closed. Textual variants, because of the multitude of textual evidences, can be evaluated and the original text more or less arrived at. We therefore can know what Scripture teaches and it wouldn't change from age to age, pace Rome, and there wouldn't be problems of incompleteness and divergences, pace Laudian Anglicanism. Scripture is autopistos, authenticating itself as the supreme authority. Can one be "infalllibly sure" of the authority of Scripture? No, in the same way as one cannot be infallibly sure of anything. But one can be sure of its infallibility, while one cannot be sure of the infallibility of any others.
So this Romanist apologetic fails because (1) it cuts all ways, (2) the authority contradicts itself often, (3) Only Scripture is autopistos, while the rest are not. We do not have to fear Rome's promotion of Sola Ecclesia, for it is a broken reed wounding all who rely on it.