Hi, Mike —
The article is an excerpt from the self-published book "His Broken Body" by Fr. Laurent Cleenewerck, an Orthodox priest, and it's an attempt to review the controversies between Orthodox and Catholics from an Orthodox point of view.
From reading a few pages of the article, I get the impression that Fr. Cleenewerck may actually have some misunderstanding about what Catholics believe regarding infallibility.
For example, in discussing the canons of the Fourth Lateran Council, he quotes Canon 3, a passage that urges secular authorities to "exterminate" heretics in their territory. Of course this is shocking, and Fr. Cleenewerck presents this as if Catholics considered it to be infallible.
However, that is probably a mistake. I'll explain why. The passage is giving directives on how to fight the influence of heresy. It says that heretics are to be excommunicated; they are to be anathematized, etc. This is all procedural direction. Infallibility applies to doctrine and not to directives. Therefore, if I understand things aright, Catholic theologians would not consider Canon 3 to be protected by the gift of infallibility.
The whole of Canon 3 can be read here in translation:
That web publication is cited in Fr. Cleenewerck's book, so apparently he was using that very translation. It was published in 1937, and it's actually a bit misleading. In particular, the word exterminate is misleading in English: a more accurate translation would be expel. So the Fourth Lateran Council was urging secular authorities to expel heretics in their territory. Admittedly, that is not up to modern human rights standards, but for the 13th century, that's how they kept the peace. It's not talking about extermination in the sense of killing, so the whole thing becomes less shocking.
That point about language leads to another question. Fr. Cleenewerck doesn't cite any source for his readers to look at the canons in the original Latin. That is really not up to scholarly standards, which is surprising, since he holds several graduate degrees.
The Amazon.com page for his book contains the book's entire index, and it has not a single index entry for any Latin terms. The on-line excerpt which I read doesn't contain any quotations from the Latin, except for the titles of documents, so I can't tell whether Fr. Cleenewerck knows Latin well enough to read the documents that he is discussing.
His resume/curriculum vitae online does not list Latin among the languages that he knows:
So all in all, I would have to say that the book has some weaknesses, and probably is not the best resource for understanding both sides of the theological divide in the depth that the subject deserves.
I hope this helps.