This is why the Protestant approach leads to heterodoxy.
I don’t think you can make that claim from only the foundation laid so far. All we have talked about is infallibility, and certainly Reformers have their views on it too, including their own view of what is or is not infallible. I don’t think we are there yet.
Notice, however, that science also denies infallibility but it does not, in the long run, lead to heterodoxy. Rather, science has had a pretty good track record of iterating toward a reliable understanding of the world, at least in some areas such as physics.
If I were to submit a scientific paper using my own value for Planck’s constant on the idea that science holds nothing infallible, I think we could both agree that I would suffer serious intellectual pounding come the peer review. That’s because I’m mixing the concept of infallibility with the concept of truth. Science doesn’t say we hold this
infallible, but once they have put it through the ringer of the scientific process, they know it is
true. This would be similar to my example above re: man existing out my window. I know it’s true independent of, perhaps in addition to, an infallible source. Science knows what Planck’s constant is without an infallible source because they are not the only way to know truth.
As to denying truth itself independent of infallibility, I believe it can and has lead to heterodoxy, but sometimes such heterodoxy is beneficial for science. Ptolemy v. Copernicus and Galileo for example.
So it is not the case that denying infalliblity necessarily leads to a multiplicity of opinions, i.e. to heterodoxy.
I don’t think it mandates it because heterodoxy demands an additional view of truth, where each is promoted as (the) truth. One could deny the Bible as infallible, and by doing so they are denying the truth of it and in it. Then they could go watch football and there would be no heterodoxy. It is only when one tries to proclaim a different “truth,” that the heterodoxy emerges.
That’s not the usual meaning of infallibility. …the infallible source helps us to determine the truth of the claim. But we already have some concept of truth apart from the infallible source.
I contend you are confusing redundant methods of proving a truth with what an infallible source really is. The infallible source doesn’t “help” define truth, it DOES define truth. It doesn’t make “statements about the truth,” it only gives the truth. If you have another way of knowing the truth of something (is the stove hot? I have a thermometer , let me check.), the results will always match the infallible source (by definition). When you don’t have that other way to know, the infallible source is what defines truth, but defining isn’t conditional on other sources; it always defines.
Did God give us the Church?
The answer to that depends on if one thinks the Bible + Teaching + Tradition are infallible or not. If you do, the answer is yes, because that is what all of those sources declare. Other faiths do not hold T + T as infallible, but do hold the Bible as such. They have a different method and hold that as their infallible way to extract the truth from the infallible source. I’ll leave it to others more qualified than I to argue the merits of each.
Would God give us a flawed Church?
If He is not just and not holy, He could. It would be His arbitrary decision. If He is just and holy, He could not.
Can we evaluate and test the claim that God gave us the Church without relying on the claims of the Church?
No, unless you can find a way to know a metaphysical truth without a revelation. The only way we know we have any church at all is because God revealed it to man. All infallibility rests upon God, as He is the only infallible source they can come from.
Or is it a matter of faith that, once accepted, entails the rest?
This is why you have to first settle the matter of infallibility. Once that is settled, you can know the truth where before that you cannot. Then it isn’t a matter of acceptance as to the truth of something. It’s a matter of something IS true, and will you accept truth or error.
…must we suppose that the Bible is infallible truth?
If we establish God’s existence and qualities, and we have a record of His revelation to us, what else can we do with it except presume it be infallible? If God gave us a lie, it would undo the entire logical sequence used to establish Him in the first place.
I am not qualified to speak on Islamic beliefs, so rather than charge an entertainment fee, I’ll leave that question to others.

