That their continuity down to this day, and the vigour of their operation, are two distinct guarantees that the theological conclusions to which they are subservient are, in accordance with the Divine Promise, true developments, and not corruptions of the Revelation.

Moreover, if it be true that the principles of the later Church are the same as those of the earlier, then, whatever are the variations of belief between the two periods, the later in reality agrees more than it differs with the earlier, for principles are responsible for doctrines. Hence they who assert that the modern Roman system is the corruption of primitive theology are forced to discover some difference of principle between the one and the other; for instance, that the right of private judgment was secured to the early Church and has been lost to the later, or, again, that the later Church rationalizes and the earlier went by faith.

On this point I will but remark as follows. It cannot be doubted that the horror of heresy, the law of absolute obedience to ecclesiastical authority, and the doctrine of the mystical virtue of unity, were as strong and active in the Church of St. Ignatius and St. Cyprian as in that of St. Carlo and St. Pius the Fifth, whatever be thought of the theology respectively taught in the one and in the other. Now we have before our eyes the effect of these principles in the instance of the later Church; they have entirely succeeded in preventing departure from the doctrine of Trent for three hundred years. Have we any reason for doubting, that from the same strictness the same fidelity would follow, in the first three, or any three, centuries of the Ante-tridentine period? Where then was the opportunity of corruption in the three hundred years between St. Ignatius and St. Augustine? or between St. Augustine and St. Bede? or between St. Bede and St. Peter Damiani? or again, between St. Irenseus and St. Leo, St. Cyprian and St. Gregory the Great, St. Athanasius and St. John Damascene? Thus the tradition of eighteen centuries becomes a collection of indefinitely many catenae, each commencing from its own point, and each crossing the other; and each year, as it comes, is guaranteed with various degrees of cogency by every year which has gone before it.

Moreover, while the development of doctrine in the Church has been in accordance with, or in consequence of these immemorial principles, the various heresies, which have from time to time arisen, have in one respect or other, as might be expected, violated those principles with which she rose into existence, and which she still retains. Thus Arian and Nestorian schools denied the allegorical rule of Scripture interpretation; the Gnostics and Eunomians for Faith professed to substitute knowledge; and the Manichees also, as St. Augustine so touchingly declares in the beginning of his work De Utilitate credendi. The dogmatic Rule, at least so far as regards its traditional character, was thrown aside by all those sects which, as Tertullian tells us, claimed to judge for themselves from Scripture; and the Sacramental principle was violated, ipso facto, by all who separated from the Church,—was denied also by Faustus the Manichee when he argued against the Catholic ceremonial, by Vigilantius in his opposition to relics, and by the Iconoclasts. In like manner the contempt of mystery, of reverence, of devoutness, of sanctity, are other notes of the heretical spirit. As to Protestantism it is plain in how many ways it has reversed the principles of Catholic theology. – from Essay on the Development of Doctrine (1845)