No matter what we actually say, we all behave as if Sola Scriptura is true. This is obvious in that there is a qualitative difference how we treat the words of Scriptures versus that of the words of ecclesiastical documents or theologians.
For any piece of ecclesiastical document or writing of a past Father, it is always a possible move to say that “Well, he’s wrong”, while this move is categorically denied to the Scriptures. That is, it is impossible to say that what the Scriptures says is wrong, while it is always a possible move to say that some theologian or church document is wrong. And of course, if for any Father or Church document, it is possible for us to judge that it has erred, then they are by definition not infallible, that is impossible for it to err, but fallible instead.
Of course, the first move of a high church Christian, when one cites some church document or Father which contradicts his claims, is always to ask what does this piece “really” mean, often a rather tedious exercise in logomachy and nitpicking (and a somewhat ironic activity in that the appeal to traditional documents is meant to resolve problems of interpreting the Bible now ends up having interpretative difficulties of their own!), with very little returns to show for it.
But eventually when one is truly backed into a corner, like in the case of St Thomas Aquinas or St Bernard of Clairvaux denying the infallible dogmas of the Immaculate Conception, they would then admit that they were in fact wrong.
In short, the phrase “the Church is infallible” is a somewhat vacuous statement without identifying exactly where this infallible Church is. Councils have arisen throughout the history of the Church and have been revoked by subsequent councils, the Fathers have contradicted each other and contradicted councils, and canonised Doctors of Theology have been known to err, even by their own standards.
Perhaps the only consistent claim to infallibility is that of Papal infallibility. While of course we may grant that there are certain times in the Church of Rome’s history whereby a dogma is clearly infallible, such as the Marian dogmas and the definition of infallibility itself, the ambiguities sets in beyond these borders. Some extend the infallibility to “ecumenical councils”, or to “certain” documents of the teaching Magisterium (although which documents depends upon a host of mutable conditions which shifts from Catholic theologian to Catholic theologian), others simply restrict the infallible pronouncements to the Marian dogmas and papal infallibility.
In short, the “Church” which is supposedly infallible, the patrum consensus or sacred tradition which is supposedly a source of divine revelation, are entities which boundaries are constantly gerrymandered to suit the theological conclusions of their interlocutor. The Church is infallible, except when it is not, we believe the patrum consensus, but there is no consensus as to what they are. We demand that Protestants prove that the Fathers taught this, except for when the Fathers didn’t mention our doctrine in which case it “developed”. Rather than admit outright that councils, canonised theologians and Fathers can and have erred and have contradicted their present teachings, or that their present teachings can’t be found in the early church, the high church advocate would rather take the challenger on a fantastic merry go round of trying to reconcile their writings and thoughts with their present claims and only when finally backed into a corner (which occurs only after the most tedious examination and arguments about their texts), they would then simply shrug and admit that they were fallible and wrong but their claims still hold!
Functionally, we are all Sola Scripturists, in that we all do believe that the Scriptures alone are infallible, everything else is fallible, and can and have erred. A council, Father or canonised Doctor of Theology is right, not because they are a council, Father or canonised Doctor of Theology, but because they speak the truth. To have been taught by an ecclesiastical council or theologians does not by itself prove one’s theological conclusions, at most it can tell us what a particular church believed at a particular moment. However Truth itself is always the judge of the Church, never the other way round. Protestants are saying what everyone is already practising.
This point, ironically, has already been noted by St Thomas Aquinas himself who wrote:
It should be noted that though many might write concerning Catholic truth, there is this difference: that those who wrote the canonical Scripture, the Evangelists and Apostles, and others of this kind, so constantly assert it that they leave no room for doubt. That is his meaning when he says ‘we know his testimony is true.’ Galatians 1:9, “If anyone preach a gospel to you other than that which you have received, let him be anathema!” The reason is that only canonical Scripture is a measure of faith. Others, however, so wrote of the truth that they should not be believed save insofar as they say true things.
-Thomas Aquinas, Lectures on the Gospel of John, ed. P. Raphaelis Cai, O.P., Editio V revisa (Romae: Marietti Editori Ltd., 1952) n. 2656, p. 488.
But of course, one can always retort that St Thomas Aquinas is here wrong. Again.