In light of my previous post, someone asked me this.
"What's changeable? They say Pope Francis has or is going to change the Church's teaching on X, Y, or Z..."
First of all - never believe anything the media says. So, having said that, let's deal with the what's changeable part.
How do we know what the Church teaches?
Consult the Catechism first, the 'CCC' put out by JP II in 1992 (revised 1997). It is the most recent catechism given to the whole Church, therefore it is the most authoritative general document in the whole world. It beats the Catechism of Trent (aka The Roman Catechism, aka The Catechism of Pius V) because it takes it into consideration. Huh? What I mean is that later documents are usually clearer than older ones because they have them in mind. Earlier ones do not clarify later ones in the same way. The CCC is a priceless treasure.
And yet, it is a rather complicated document. No, it is not easy to understand all that it contains. Also, it does not go into too many specifics, that is, if you want to get into the finer points of some issue, like bioethics, or Christology, etc. So we need more.
Some of the more detail is supplied - as authoritatively - in papal encyclicals - the newer the better, again, for the reason I gave above. If you want more info. on life issues, consult JP II's Evangelium Vitae, on contraception, try Humane Vitae of Paul VI, etc.
The higher the authority the less wiggle room to say that the Church doesn't teach X or doesn't teach X exactly, etc. If your bishop says X, that's one thing - a strong thing. If a Papal Congregation says it, so much the more. If the pope says it, so much the more. If he says it in a official and specific way, it is practically certain in every respect.
Also, is the matter of central important or not? Is it about God and or salvation? Is it sort of, is it not really at all? The Church has no teaching on the Republican Party of the US. It does say that pro-life issues are extremely important and therefore you can conclude that you should vote for a pro-life party rather than a pro-abortion party. But what if the party is pro-life but racist, war-mongering, polluting, etc., etc.? Well, that requires some discernment. In 1930s Germany your choice was between Nazis and communists. Nice choice.
How do we know if a teaching is changeable?
Is it taught at a high level? Is it about a matter of central importance? Is it about an objective evil?
Let's compare the celibate priesthood to abortion.
Abortion has been condemned since the Didache, which might be older than certain New Testament texts. It was condemned overtly many times by the highest authorities in ordinary ways and in extraordinary ways, the most powerful being JP II's Evangelium Vitae.
Is it about an objective evil? Abortion is murdering innocent pre-born life. One cannot imagine a circumstance when the essential aspect of this can be taken into a new light.
On the other hand, priest could marry in the Ancient Church, they can still marry in certain Catholic rites now, and convert Anglican priests are often ordained today.
Is it about an objective evil? Marriage is a good thing in and of itself.
Let's talk about a few topical matters:
1) The genetic manipulation of fertilized human cells so that they have three parents. "All children deserve to be raised by a mother and a father," said Pope Francis recently, echoing a rather straight-forward bit of common sense. The Bible doesn't mention this, and it doesn't need to. (See how Protestantism is a weak link?)
2) Re-marriage after an actual marriage has taken place. Bigamy is sinful. It is a form of adultery. This fact can never be changed. The Bible does mention this, and does not need to.
3) Gay 'marriage.' In the beginning God created them male and female... The Bible doesn't really need to mention this, but it sure helps. What is the point of sex, of marriage, of love?
So what's changeable?
Not the moral law. These are based upon the nature of things, the world that God has created. new situations arise, and we have to learn to apply the moral law in new ways, as in the case of genetic manipulation of the human person. The laws, the principles do not change.
Rules of discipline may change. For instance, clerical celibacy, use of the vernacular in mass, new rules for religious congregations.
Let me close by talking about an example that is a little bit difficult: usury. The Medieval Church vehemently condemned lending money at interest (hence the usefulness of Jews). Now it says it is not absolutely forbidden. Is this a change of the Church's teaching on an important matter of morality? Looks like it. It's not an inconsequential matter for sure. But let me say this, the Medieval agrarian, exchange economy and the contemporary industrialized, monetary economy are very different. Further, in the Middle Ages we had one religion dominating the economy and able to set a single set of rules for all. Are these rules sufficient to warrant this turn about? I say yes, but not in the sense that usury is no longer of moral concern to Catholics. So what changed - the Church or the circumstances? It looks like the circumstances.
Can we say, then, that when it comes to something like AIDS, the Church ought to change its teaching on contraception? I would say no, because there we are talking about something that cuts right at the heart of the meaning of sexual love: sharing life and creating life. Contraception undermines that in its very essence. It's easy enough to argue our way out of this. I don't find AIDS in Africa a very compelling argument. Contraception encourages people to take risks. Would you have sex with someone 100 times, if the success rate of the condom was 99%? Would you put a gun to your head that had a hundred chambers, only one of which had a bullet in it? How many times would you pull the trigger? We can illustrate the success rate of the use of contraceptives by pointing to the fact that a great number of abortions (I read the stat recently but can't recall it right now) happen after contraception was used. Too bad you can't abort AIDS, eh?
Of course, the future is wide open and scary. I don't know what the future may bring. I hope that those many freaky dystopian prognostications of sci-fi novels and movies don't come true, but if they do I have faith that the law of God revealed in Christ will be sufficient to supply an answer to life's complexities even then. It has so far.
Basic humanity stays the same, thus, the principles of Catholic morality stay the same. You can learn what these are most authoritatively from the Catechism, from papal encyclicals, from the documents of the ecumenical councils.