It's no secret that Catholicism is a constantly evolving religion. Its beliefs have changed time and time again since its origins around the 3rd-5th centuries, resulting in modern Catholicism being radically different from Biblical Christianity, the teachings of the early Church, and even the beliefs and practices of ancient Catholicism. And you don't have to be a "Protestant" to see this. In the 19th century, John Henry Newman observed this glaring problem, and came up with the theory of Development of Doctrine to explain it.
First, it should be noted that Development of Doctrine is a theory by a Catholic theologian. As far as I am aware, it is not an official doctrine of the Catholic Church. As such, Catholics are free to draw their own conclusions on it. However, when the historical evolution of the Catholic Church is brought up, Catholics often mention it. I was even once mocked by a Catholic for showing the Catholic Church has changed over time because, in his exact words, "apparently, you haven't heard of Development of Doctrine".
So, what is Development of Doctrine? The term was first introduced in a book entitled "An Essay on the Development of Christian Doctrine" by John Henry Newman. In it, Newman attempts to defend the idea that the Catholic Church's beliefs have remained consistent throughout history. However, they have become more explicit, and better defined, over time. According to Newman's argument, doctrines that the Catholic Church seems to have added over time actually follow a similar principle to doctrines that "Protestants" would accept. The Trinity, for example, is not explicitly taught in the Bible, yet "Protestants" would agree it is true. [Side note: A major flaw in the term "Protestant" is that no, not all so-called "Protestants" would agree.] By Newman's logic, doctrines "Protestants" reject, such as Purgatory, are also found in the scriptures, yet the Catholic Church was necessary to make them explicit.
The first major problem with this is that the Trinity is explicitly taught in the scriptures. If you, as a Christian, cannot see that the Trinity is in the Bible, the flaw is on your end, not in the scriptures. To really illustrate this point, consider that at no point does your average "Protestant" apologist need to resort to extra-Biblical sources to defend the Trinity. We don't need to argue from the Church "Fathers", the words of a Reformer, a council, the only thing I would need to show, for example, a Mormon, that the Trinity is a Biblical fact is the Bible itself. And I'm not even a pastor. I'm just some dude on the internet. If I can show that the Trinity is a Biblical fact without even once referring to an extra-Biblical source, obviously it is explicitly taught in the Bible.
But what about Purgatory? Is it in the Bible? There are some verses a Catholic might try to argue do show Purgatory. But the fact is, these verses do not show anything like the concept of Purgatory. And actually, a lot of the Bible indicates that Purgatory is a myth. For example, Paul says that, for a believer, to be absent from the body is to be present with Christ (2 Corinthians 5:6-8; Philippians 1:23). I would prefer not to turn this article into a full refutation of Purgatory, but suffice to say that there is a major difference between the Trinity, which is clearly taught, and Purgatory, which is clearly not.
The second problem is that other religions could easily use it. And in fact, they do. There are no supposedly Christian religions that do not claim, in some way, that their doctrines are not new, but merely more explicit. If you try hard enough, you can make the Bible say anything you want. For example:
"And this shall be the plague with which the Lord will strike all the people who fought against Jerusalem: Their flesh shall dissolve while they stand on their feet, Their eyes shall dissolve in their sockets, And their tongues shall dissolve in their mouths." (Zechariah 14:12, NKJV).
Behold! The Lord will cause a zombie apocalypse! What is a zombie apocalypse? Well, it is a plague, is it not? Zombieism is a disease that spreads at a very rapid pace, causing mass fatalities. If these people are standing on their feet, why is their flesh dissolving? Even their eyes and tongues! This doesn't just happen to living people. This verse is clearly talking about the walking dead!
Except it's not, is it? I'm not going to waste time refuting that interpretation, since I doubt anyone reading this article takes it seriously, but the fact is, if there was a religion out there that took this particular view of eschatology, they can cram it in. Crazier doctrines have been forced upon the scriptures (including by the Catholic Church).
In fact, Catholics must agree with this concept. See, to them, they're the only ones not doing it. It is easier to defend, for example, Sola Scriptura, than to defend an impending zombie apocalypse, because Sola Scriptura is true. And yet, a Catholic will vehemently disagree. A Catholic believes Sola Scriptura was invented during the Reformation, and then retroactively forced upon the scriptures. In every discussion with Catholics regarding Sola Scriptura, I use the scriptures almost exclusively. I might occasionally switch to Augustine, just to confuse them, but I usually stick to the scriptures.
I could claim Development of Doctrine here. Sola Scriptura is clearly taught in the scriptures, yet obviously, it wasn't called Sola Scriptura back then, and I'm actually more inclined to reject even the term "Sola Scriptura" in favour of the term "the Sufficiency of Scripture". Yet, in spite of the overwhelming Biblical evidence, which isn't a term I use lightly, Catholics will still insist that this is a doctrine that was made up during the Reformation. If such a solid principle as Sola Scriptura can be rejected as an "innovation" of the Reformation in spite of such Biblical evidence, how are we supposed to take the Marian dogmas, or the 7 sacraments, or the Papacy, none of which have any basis in scripture, as merely being "more explicit"?
The theory of Development of Doctrine is really one of those theories that backfires. It acknowledges a major problem for the Catholic Church, namely that their dogmas are simply made up over time, by attempting to solve it. Yet, unfortunately for Catholics, it fails miserably. The fact is, Catholicism is a constantly evolving religion. Throughout history, the Catholic Church has removed some Christian doctrines, diluted others, and added a wide range of its own. Some of these doctrines are not even new, but actually stolen from other religions. This not only erases the argument from historicity, but also shows that the Catholic Church is no more valid than the various cults that arose in the 19th century. According to Jude, the brother of Jesus (in stark contrast to the Catholic dogma that Mary was a perpetual virgin, and thus could not have given Christ any siblings), Christians are to contend for the faith that was delivered to the saints once for all. This is quite the opposite of the Development of Doctrine, which is really just an excuse for why Catholics have been contradicting both the Bible, and even themselves, for 1,700 years. If any doctrine evolved after 95 A.D., that doctrine is false. Catholicism evolved after 95 A.D., therefore Catholicism, by the tacit admission of John Henry Newman, is false.