CathInfo.com

Here's Salaverri:

This dispute is due to the error of attempting to apply the notion of material heresy to those who profess false religions, i.e., the claim that Protestants or Orthodox and the like can be "merely material" heretics ... with the equally-erroneous corollary that non-members of the Church can still be within the Church and therefore saved.

I doing so, they confounded the meaning of material heresy, claiming that it could apply to those in formal error so long as they were "sincere" in their error.

Others redefined the term so that material heresy no longer existed, claiming that heresy of its nature can only be formal, while so-called material heresy isn't heresy at all.

In reality, FORMAL heresy pertains to those who do not believe what they believe with the correct formal motive of faith, i.e. based on the teaching authority of the Church.  MATERIAL heresy is the holding of an objectively heretical proposition due to ignorance, THINKING that it was taught by the Church.  FORMALLY they believe the proposition due to the correct formal motive, believing in their ignorance that it was taught by the Church, but they are mistaken about the fact of whether it was taught by the Church.  THAT is the correct distinction that got warped into "sincerity" vs. "insincerity".

It was obvious to me what Struthio meant. When he said a manifest heretic "may not be a heretic," he meant a public heretic who may not have the pertinacity of will to be culpable as a formal heretic, and may not be a heretic internally if you will, but only exteriorly and publicly.

You also then cling to the erroneous notion of the formal vs. material distinction, where formal heresy is tied to sincerity and culpability, the internal vs. external forum.  This is simply wrong.

Father Jenkins calls the letter "earth-shattering".

Yes, I watched this excellent video a few days ago.  Fr Jenkins also brings up his concerns over the Trump letter due to the Naturalism contained in it.