John Henry Newman, answered from the texts and cited to the page.
The question is one I spent the better part of my intellectual life trying to answer precisely, and I shall try to be exact rather than merely eloquent. The historical fact is plain enough: the body of doctrine known as Catholic, professed today by Eastern and Western Christendom alike, is undeniably the historical continuation of what bore the name Catholic in every preceding century back to the first.1
But historical continuity alone does not settle the matter. The real question is whether modern Catholicism is logically, as well as historically, the representative of the ancient faith — whether what has been added is the legitimate growth and complement of the original deposit, or something that has displaced it.2 My answer is that it is the former: the natural and necessary development of the doctrine of the early church, its divine authority included in the divinity of Christianity itself.
Now the objection presses immediately, and I do not wish to soften it. It may be said that a development can be natural and yet untrue to its original, as diseases come of nature yet are the destruction of health; that the very causes which stimulate the growth of ideas may also disturb and deform them.3 This is what may be said, and I acknowledge its force.
It becomes necessary in consequence to assign certain characteristics of faithful developments — marks which none but faithful developments possess — and to distinguish them from corruptions.4 A corruption, taken literally, attaches only to organised living matter; a stone may be crushed to powder but it cannot be corrupted. Corruption is the breaking up of life preparatory to its termination, the reversal and undoing of what went before, the loss of vigour and powers of nutrition, assimilation, and self-reparation.5
An idea that is genuinely developing does the opposite: it retains one and the same type, the same principles, the same organisation; its beginnings anticipate its subsequent phases, and its later phenomena protect and subserve its earlier; it has a power of assimilation and revival, and a vigorous action from first to last.6 There is one further thing I would add, because it is easily missed.
Refusing to follow the course of doctrine as it moves on is itself a form of corruption, not a safeguard against it.7 The Sadducees who denied what was covertly taught in the Book of Exodus, the Samaritans who refused to add the Prophets to the Law — these were in appearance only faithful adherents to the primitive doctrine. Our Lord found His people precisians in their obedience to the letter; He condemned them for not being led on to its spirit, that is, to its developments.8
The Gospel is the development of the Law, and what difference seems wider than that between the unbending rule of Moses and the grace and truth that came by Jesus Christ? Yet no one calls that a corruption. And if developments are genuine — if certain great truths and duties naturally and legitimately result from the doctrines originally professed — then it is but reasonable to include those true results in the idea of the revelation itself; and if the revelation be not only true but guaranteed as true, to anticipate that they too will come under the privilege of that guarantee.9 Development and infallibility are not in tension; the one is the argument for the other.
That faith is undeniably the historical continuation of the religious system, which bore the name of Catholic in the eighteenth century, in the seventeenth, in the sixteenth, and so back in every preceding century, till we arrive at the first.An Essay on the Development of Christian Doctrine
The only question that can be raised is whether the said Catholic faith, as now held, is logically, as well as historically, the representative of the ancient faith.An Essay on the Development of Christian Doctrine
the causes which stimulate the growth of ideas may also disturb and deform them; and that Christianity might indeed have been intended by its Divine Author for a wide expansion of the ideas proper to it, and yet this great benefit hindered by the evil birth of cognate errors which acted as its counterfeit.An Essay on the Development of Christian Doctrine
it becomes necessary in consequence to assign certain characteristics of faithful developments, which none but faithful developments have, and the presence of which serves as a test to discriminate between them and corruptions.An Essay on the Development of Christian Doctrine
Corruption, on the contrary, is the breaking up of life, preparatory to its termination... the body has a function of its own, and a direction and aim in its action, and a nature with laws; these it is now losing... its vigour and powers of nutrition, of assimilation, and of self-reparation.An Essay on the Development of Christian Doctrine
There is no corruption if it retains one and the same type, the same principles, the same organization; if its beginnings anticipate its subsequent phases, and its later phenomena protect and subserve its earlier; if it has a power of assimilation and revival, and a vigorous action from first to last.An Essay on the Development of Christian Doctrine
one cause of corruption in religion is the refusal to follow the course of doctrine as it moves on, and an obstinacy in the notions of the past.An Essay on the Development of Christian Doctrine
Our Lord found His people precisians in their obedience to the letter; He condemned them for not being led on to its spirit, that is, to its developments.An Essay on the Development of Christian Doctrine
if there are certain great truths, or duties, or observances, naturally and legitimately resulting from the doctrines originally professed, it is but reasonable to include these true results in the idea of the revelation itself... if the revelation be not only true, but guaranteed as true, to anticipate that they too will come under the privilege of that guarantee.An Essay on the Development of Christian Doctrine