Promoting or practicing the Mosaic Law today makes vain the Cross of Christ, which alone has any saving or propitiatory power (power to forgive sins).
It's not a small heresy!
It also happens to be the very first major heresy in the Church. You kept characterizing his view as material heresy. I would say more that it's objectively heretical. I know what you're trying to say, but using the term material actually precludes the possibility that it's formal (they're opposite terms).
And formal heresy does not equate to "sincerity", something in the internal forum that cannot be judged publicly. If it has become manifest that someone is pertinacious in the belief, and persists in it after it has been shown to him that the Church has condemned it, then it can be judged in the external forum to be formal heresy. Heresy goes from material into formal as soon as one even implicitly rejects the authority of the Church behind the teaching, since the authority of the Church teaching is the formal motive of faith (that's the argument I was having with Drew on the other thread). That's the reason for the saying that if you reject one dogma, you reject them all. That's because in rejecting one of them you are rejecting the teaching authority that's behind them all, and whatever ones you do still believe, you are not longer believing with the formal motive of faith, the authority of the Church, but just because you have found them agreeable based on your own private judgment.
This transmogrification of the term "formal heresy" into insincerity is yet another product of the phenomenological subjectivism that Bishop Williamson has always rightly railed against. I can be as sincere as anyone can be, but if I do not believe the dogmas of the Church based on the teaching authority of the Church, I am a formal heretic. Material heresy can only be due to ignorance regarding what the Church teaches (cf. Saint Augustine). And the outward sign, again according to St. Augustine, of heresy being only material is the immediate willingness to change one's mind after being corrected and the ignorance of what the Church teaches has been removed. Now, it's more complex with truths that have not been directly defined, because one might BELIEVE that his propositions (logical conclusions drawn from defined truths) are consistent with Church dogma, and hold to them based on this assessment. Someone else might then come along and dispute it, arguing that it does in fact contradict Church teaching. So the dispute there is about whether the argument is sound. If the person drawing the false conclusions were corrected by proper authority, who could definitively rule against his opinion, he would show that the heresy or error was only material by willingly retracting his prior opinion. Problem is that when it's a dispute among peers, the person holding the opinion might conclude that the rebuttals are wrong without necessarily compromising their willingness to yield to the Church.
That's why the V2 errors are more slippery. Papal claimants prior to Francis claimed that these teachings WERE in conformity to Tradition and felt that they could be reconciled to the past with a "hermeneutic of continuity". That suggests that they care about whether these propositions are reconcilable with Tradition. Bergoglio on the other hand has made statements about how what he's saying is probably heresy, but then went on as if he didn't care.
So, in short, the difference between formal and material heresy is that material heresy pertains to WHAT you believe, while formal heresy speaks to WHY you believe it. Now, some truths are necessary to believe even materially, so that there cannot be supernatural faith without believing in those things ... usually that God exists and is a Rewarder along with basic belief in the Holy Trinity and the Incarnation. Without those basic things, as St. Thomas teaches, there can be no supernatural faith, not faith with the supernatural motive. You can't have form without some matter in this case.