I can't speak to his suggestion, since perhaps he knows some Catholics who do "have trouble" with his supposition. But his supposition is baseless.
In the first place, his supposition seems to presume that Catholics attribute all disunity to sola scriptura: that there are no other causes of disunity among Christians. This idea is false, and a Catholic would be mistaken to make such an attribution, given the Church's history. Without trying very hard, and without being an expert in Church history, I can think of a few counter-examples:
- Political, as when (for example) Barbarossa propped up antipopes for himself
- Philosophical, as when Arius denied the deity of Christ
- Theological, as when the Orthodox deny the Filioque
In the second place, and as I have just illustrated, the same effect can have multiple causes. A more mundane example: let's suppose I burn my finger. There are lots of ways I could do that:
- Touching a hot stove
- Dipping my finger in boiling water
- Taking the radiator cap off of a hot engine
But perhaps by thinking about my burnt finger, we can see something about the other case. My finger actually gets burned because it comes into too close proximity to something that is hot. That, of course, is the unifying idea behind the various ways I might burn myself: too much heat, too close to my skin (however that might happen).
Similarly: schismatics could be said to have in common the fact that they all refuse to submit to the authority of the Church. The schismatic who deliberately rejects the Pope in order to live by sola scriptura is in principle not particularly different from the schismatic who deliberately rejects the Pope in favor of some pretender: he has rejected valid authority.
[Note: I am not suggesting that today's Protestants are necessarily formal schismatics].
Now there may be other consequences of a specific form of schism. The man who prefers a false pope isn't as likely, for example, to make up his own creed: he will leave that to his "pope" if it happens at all. The Protestant case is obviously and necessarily different, as we may readily see from their history and from the variety of their creedal statements. But in each case, the Protestant and the "Catholic" schismatic has at some time or other arrived at the same point, where he refuses to submit to the Pope on matters of faith and morals.
How this is supposed to be a difficulty for the Catholic is mystifying.
What is even more baffling is this question our blogger asks: "why besides blatant exercise of the post hoc ergo propter hoc fallacy would one blame the large number of denominations on Sola Scriptura?"
Well, let's see. Presbyterians and Baptists (for one example) are denominationally distinct because of doctrinal differences. How did they arrive at their doctrinal distinctives? By appeal to Scripture. Yet both groups hold to sola scriptura. If they shared a common authority by which doctrinal disputes might be settled - like, for a totally random example, say, a Pope - and if both groups were committed to accepting that Pope's doctrinal declarations as definitive, where would their disunity come from?
There wouldn't be any.
As it is, though, they are committed to a law book without a judge to interpret it for them. The upshot is that they must interpret it for themselves. And because they hold their own interpretations as binding upon themselves, they feel perfectly willing to sever the bonds of fellowship with those who do not agree with them sufficiently - that is to say, with those who do not agree with them about some doctrine or doctrines understood to be "essential."
This is not a post hoc fallacy. This is how it has happened throughout Protestant history. This is why the Bible Presbyterians split off from the OPC, for one example: disagreements about alcohol consumption and dispensationalism. Both the OPC and the BPC splinter were totally committed to sola scriptura - and their doctrinal differences drove a wedge between them.
Now imagine (yes, this takes some effort, considering the parties) that the BPCers and the OPCers were both willingly bound to submission to an authority - and once again just for grins let's call that authority "the Pope" - their split would never have occurred. Because they could appeal to that authority. And they would be content to accept the decisions of that authority.
Sola scriptura demands of God's Word what it is not meant to do. It does not interpret itself. It does not apply itself. It should not be surprising that bad results are the consequence of misusing a thing. Disunity is a natural and inevitable consequence of sola scriptura. No post hoc appeals required.