The first part says essentially, that all human institutions must change, (let’s call this “rule 1”) and that it was “inevitable” that Christianity would change.
No, not institutions, ideas. And not just human ideas, even divine ideas would necessarily be affected by the minds in which they reside. For better or worse.
So you have admitted that the gospel has changed from its original state. We call that “apostasy”
No, I have not stated that it has changed, but that it has developed. As in the parable of the mustard seed, that which was in the beginning is smaller than what it later becomes.
Perhaps it is more precise to say, that doctrine has become “unfolded” over time. Indeed, all Revelation was contained in the person of Jesus Christ, but not all of Revelation is easily grasped by the human mind. That is what I mean by development - the faith grows; the Spirits leads us into all truth (of which the Church Christ instituted is the pillar and foundation).
Apostasy is rejecting the Faith.
Heresy is believing in un-natural (i.e. not according to the nature of the Christian revelation of Jesus Christ) developments. As in the human body, not all developments are for the good. Tumors are developments, but they are destructive and not good for the organism. But if no development took place at all, the Church would still be an infant.
The earliest Church may not have believed in the Trinity. But then again they certainly didn’t disbelieve in it either. The Trinity was implicitly contained in what they believed, but it had yet to be unfolded, to be explained and stated clearly.
Since someone else disagreed with me, that doctrines develop, I should like to note that I speak solely as a lay-person and present one of the traditions within the Church, namely that of John Henry Cardinal Newman. Some people do not agree that doctrines develop and indeed, I may be wrong here. I try to give our critics the benefit of the doubt, and play along…
"Christ is indeed the fulness of God’s revelation, but Christianity developed - it had to - because it was still an infant. Christianity was an idea, and it was a living idea. On every idea that a man holds, he will excercise the power of thought. When thought is excercised on an idea, it can most certainly not stay the same (in a strict sense), but rather it must develop."
And yet this “development” was OK because you have an infallible pope guiding you.
I hadn’t jumped to that conclusion yet

. I merely noted, that development is inevitable. Some development will be true to the original idea, some will not. Indeed, all development
might be false, if we were left to ourselves.
I do not, however, think that God would give us his Revelation and then just leave it to decay.
But you only have an infallible pope if Catholicism is true.
That is the other way round… Catholicism is true only if the pope is infallible. I have at no point pre-supposed that Catholicism is true.
What I argue is, that an infallible interpreter is necessary, and that one is to be expected to arise after Christ ascended to Heaven. I then noted that Peter and the bishops of the NT act exactly the way I would expect an infallible guide to act, and this makes sense, since it is to those that Jesus promised the Holy Spirit to guide in all truth. Unfortunately, eventually Peter died, leaving his office vacant. But an interpreter is still necessary. Now, I wonder if there are any likely candidates for his office in Church History, and lo and behold I find that from very early on, the bishop of Rome, the successor of Peter, and the bishops who claim succession also from the apostles, are acting with extraordinary authority in exercising exactly this role in the Church. Condemning heresy, defining orthodoxy, protecting the sheep from the wolves and acting as the visible bond unity of the Church.
So Catholicsm is a self-referring infallible system that evolves over time, and it is true because it says it is.
The pope is infallible because there really are no other candidates for infallibility. I never made your circular argument.
And I guess that pope is not subject to rule 1, which is that all institutions must change, because he is an “external authority”.
The Pope may
hold a heretical doctrine, but he is by a special charism of the Holy Spirit unable to
teach error. The Popes role is ultimately not about making the development, but judging the development. “What you bind on earth shall be bound in Heaven…”
In order to judge which development was good and what was bad, it is neccesary that there exists an external infallible authority. This is exactly the role that the Papacy has been playing ever since the first pages of Acts.
So the pope is not Catholic?
External meaning here that the “inner guidance of the Holy Spirit” is not sufficient as history shows us quite clearly. There must also be an external, that is a visible, authority.
On one hand it is a very Mormon way of looking at the world.
On the other hand, if I made the same argument, you would laugh me off the boards. Any doctrinal change is ok because it was made by an “infallible external authority”. So we kick out Arianism, the pre-existence, say that Mary was Immaculately Conceived
Indeed if I made the same argument, I would laugh myself off the boards. But you misunderstood me on several points, and have been beating up a lot of strawmen. I hope I have made it a bit clearer now.