1. Your beef is with the Church. I did not alter the text at all
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Of course it is. The problem is that the Church makes dogmatic and absolute statements. And then contradicts them with other dogmatic statements. And then claims that there is never any conflict.
This latter claim is self-evidently false, and it causes the whole souffle to fall, leaving one with a mess.
It makes the Church sound just like Protestants when their logic fails: a retreat to authority.
And the problem with the Church's claim to absolute and unquestioned authority is that the Church murdered too many people and raped too many boys over the years to be trusted with absolute and unquestioned authority. It has lost the claim to unquestioned authority, and has to demonstrate its bona fides. Would you unhesitatingly and unquestioningly trust the babysitting of your young boy to the priests in the rectory? Not if you're a responsible parent.
So you have indeed put your finger on just exactly the problem. The Church blew itself up by murdering and raping people. It does not any longer have unquestionable authority. Its authority is questionable, and its agents are suspect, as the fruit of their own actions.
That doesn't mean that the Holy Spirit is all gone. It DOES mean that simple claims of infallible and unchanging doctrine, when they collide with obvious changes in doctrine, can't be settled by an appeal to authority, because both doctrines had authority - and also because the Church has lost absolute authority: it and its agents are not entirely trustworthy or creditable.
We are thrown back upon reason, and upon our reasonable levels of trust. Had the Church not misbehaved so terribly over the course of time, this would not be so. But the Church murdered and raped tens of thousands, and so it is so.
The Church used to say in the absolute what you said.
And now it says something different.
The Church has indeed said that nothing can be changed.
But is has indeed changed things.
To claim that it hasn't is to be dishonest: it very clearly has.
What, then?
Well, most people choose the doctrine they like best, or that seems most reasonable to them, in light of SOMETHING.
In my own case, I compare everything to things that God has manifestly done in the world, and to the specific recorded words of Jesus. Whatever corresponds to reality or to Jesus (or ideally both) is the right answer, as far as I am concerned.
Other Catholics parse things differently.
Appeals to the authority of the Church don't solve things, because I don't trust the Church enough to babysit my little boy unsupervised, and that distrust and fear has been earned by the behavior of the Church. So it's not simply a matter of saying that a Pope said thus and so in 1871. A closer look at what he said, and whether or not it makes sense is required. Otherwise one ends up with the doctrine of 1871, in conflict with the doctrine of 2017 on important points, and in conflict with one's own reason and good conscience.
Ultimately, to hold the Church together people have to accept a pretty significant degree of ambiguity, because that's what we've really got.