The famous Newman quotation, part of which, which forms the title of this post comes from the introduction to one of Newman's greatest works, an essay on the development of Christian doctrine. When he started writint the essay, he was an Anglican and by the time he'd finished it, he had proved his own conclusion by becoming a Catholic.
Meanwhile, before setting about this work, I will address one remark to Chillingworth and his friends:—Let them consider, that if they can criticise history, the facts of history certainly can retort upon them. It might, I grant, be clearer on this great subject than it is. This is no great concession. History is not a creed or a catechism, it gives lessons rather than rules; still no one can mistake its general teaching in this matter, whether he accept it or stumble at it. Bold outlines and broad masses of colour rise out of the records of the past. They may be dim, they may be incomplete; but they are definite.
And this one thing at least is certain; whatever history teaches, whatever it omits, whatever it exaggerates or extenuates, whatever it says and unsays, at least the Christianity of history is not Protestantism. If ever there were a safe truth, it is this. And Protestantism has ever felt it so. I do not mean that every writer on the Protestant side has felt it; for it was the fashion at first, at least as a rhetorical argument against Rome, to appeal to past ages, or to some of them; but Protestantism, as a whole, feels it, and has felt it. This is shown in the determination already referred to of dispensing with historical Christianity altogether, and of forming a Christianity from the Bible alone: men never would have put it aside, unless they had despaired of it. It is shown by the long neglect of ecclesiastical history in England, which prevails even in the English Church.
Our popular religion scarcely recognizes the fact of the twelve long ages which lie between the Councils of Nicæa and Trent, except as affording one or two passages to illustrate its wild interpretations of certain prophesies of St. Paul and St. John. It is melancholy to say it, but the chief, perhaps the only English writer who has any claim to be considered an ecclesiastical historian, is the unbeliever Gibbon. To be deep in history is to cease to be a Protestant.