The necessity of Doctrinal Development (hereafter known as DD) is made evident in the fact that errors cannot be built upon just as one cannot build a second story when the first already lay on a faulty foundation. When the heretics appear to progress in their doctrine, (and they often call themselves "progressive"), they are not in fact developing their doctrine; they are trading old errors for new ones. No heretical or erroneous doctrine can be seen to have developed in an organically successive manner. True DD is manifest when tradition is maintained, not abandoned and since those in error do not retain even their doctrines of yesterday (much less their starting point), they cannot be true developments.
Why then, since DD has occurred for 2,000 years (actually much longer), is the Catholic Church (if we say she alone is privy to it) not demonstrably superior to the alternatives? Or put another way, since Catholic "truth" has developed over thousands of years, how is it that an undeveloped error may seem even remotely reasonable while laid at its side? I answer in two ways:
1. The errors only have the flavor of novelty & smell as if they originate with man. In fact, they have been around from the beginning (Cain's argument is identical with a modern liberal's) and all errors originate with the father of lies in one way or another and are therefore (usually) quite clever. In summary, the errors (at least the most deceptive), while having such appearance, are at their root neither new nor find their origin in worldliness but in other-worldliness... "We do not wrestle against flesh and blood". It was not a man made lie that deceived Eve and the lie has long lost the right to be called novel.
2. There is only one truth while there are an infinite number of errors; ergo "we believe in one holy Catholic & apostolic Church". No outside group has maintained a particular error while developing it along its organic path. As soon as their error is exposed and defeated, they adopt another. Therefore, "as a dog returns to his vomit" the enemies will repeat errors which the truth has long since put asunder. So it seems the truth (the Church) is under attack from all angles all the time. This is true... all the more reason to recognize that she is the true Church. The errors have only their animosity towards truth as their common ground.
Error cannot be developed for long; only what is fully true may progress indefinitely. The truth will never run into a brick wall nor argue itself into a circle. The truth sails straight forward on an endless sea while error sails in circles close to the shore or makes a sharp left or right and immediately runs aground.
Error by its nature cannot develop (at least not indefinitely). All who have fallen victim to errors stop the development of their beliefs before they accomplish the irreparable damage they aim for. Calvin has to stop short of calling us robots, Luther has to stop short of calling post-baptismal sin meaningless, and modern liberals need to stop short of saying murder of any kind is a matter of free choice.
Reformers in general try to stop short of solo scriptura but if you challenge them on Tradition they will revert to that error. This also illustrates my point about the arbitrary switching between errors. If I told a Reformed Protestant "you don't believe in Church authority, you believe in Scripture alone" he'd say "I believe in sola Scriptura not solo Scriptura" but if I had said to the same Protestant "you deny the clear Tradition of the sacrificial nature of Christian worship" he'd say "we don't find that in the New Testament". Do you see how he trades one error for another depending on which suits his immediate need the best? Bryan Cross does.
But the point is that Catholics didn't need to stop short of calling Mary the "mother of God" nor did we need to stop short of saying she was immaculately conceived or that contraception is intrinsically evil. Right or wrong, there is a uniqueness about the Catholic insistence on development. If wrong, how is it that this doctrinal system has continuosly developed for so long without destroying itself?
Yes, there is something to be said of the uniqueness of the Catholic Church in this regard and in a host of others.