|Newman, thinking deeply|
For one thing, only its system of doctrine is the 'many-sided' object we looked at in a previous post - only this church has gone about collecting doctrines together, so that one truth supports another. For example, "The Incarnation is the archetype of the Sacramental principle*. From the Sacramental principle come the Sacraments properly so called. Of the Sacraments, Baptism is developed into Confirmation on the one hand; into Penance, Purgatory, and Indulgences on the other." Needless to say, Newman gives a few dozen examples, but these will suffice. Further, even its critics insist on the 'wholeness' and integrity of this system of teachings, if only because it means they can opt out of it entirely.
*(Newman just expects you to know what he means by this; no explanation is forthcoming. However, he's simply referring to the meaning of the word 'Sacrament' - an encounter through the senses with a divine, invisible reality. This is how God has chosen to reveal Himself to us.)
What about about other churches? They have systems of doctrine, too, after all. Could one of them be the legitimate development of the Church which Jesus founded all those years ago? The Anglican Newman is not convinced: "If ever there was a safe truth, it is this...To be deep in history is to cease to be Protestant. This is shown in the determination already referred to of [their] dispensing with historical Christianity altogether, and of forming a Christianity from the Bible alone: men would never have put it aside, unless they despaired of it."
The words and acts of the early Church speak strongly of a latent, papal authority-in-waiting. St. Clement (the fourth pope) writes to the church in Corinth, while it is without a bishop, to settle some of its internal disputes, and he was never accused of interfering in something which didn't concern him. During the many doctrinal disputes which were part of the early Church's life, Rome was the destination of deposed bishops and heretics alike, who streamed to Rome to plead their case before its Bishop, the Pope. There is no equivalent traffic to any other See, not even Jerusalem, the birthplace of the Church. Finally, there is the praise heaped upon the Roman church, and its bishop, since the earliest times, from St. Iraneaus in the 2nd century, to this from St. Jerome in the 5th: "I am associated in communion with thy blessedness, that is, with the See of Peter. I know that on that rock, the Church is built."
Still need historical proof? This is from the very end of the book, and it's awesome: "If there is a form of Christianity now in the world which is accused of gross superstition, of borrowing its rites and customs from the heathen, a religion such, that men look at a convert to it with a feeling which no other denomination raises viz. with curiosity, suspicion, fear, disgust...[if Christian communions separated from it] are but local; if they continually subdivide, and it remains one, if some of its members are degenerate and corrupt, and are surpassed in conscientiousness and in virtue by the very heretics whom it condemns;—if heresies are rife and bishops negligent within its own pale, and if there is but one Voice for whose decisions the peoples wait with trust, one Name and one See to which they look with hope, and that name Peter, and that see Rome;—such a religion is not unlike the Christianity of the first to sixth Centuries.” So...what does the theory anticipate, and the history show? "If there must be, and are in fact, developments in Christianity, the doctrines proposed by successive Popes and Councils, are they."