In 1870 the First Vatican Council defined the infallibility of the Pope under certain circumstances. In a letter dated February 5, 1871, Newman says:

My dear Fr. Coleridge,

I began to read Fr. Harper’s papers, but they were (to my ignorance of theology and philosophy) so obscure, and (to my own knowledge of my real meaning) so hopelessly misrepresentations of the book, that I soon gave it over. As to my answering, I think I never answered any critique on any writing of mine, in my life. My Essay on Development was assailed by Dr. Brownson on one side, and Mr. Archer Butler on the other, at great length. Brownson, I believe, thought me a Pantheist–and sent me his work to Rome, by some American Bishop. Mr. Butler has been lauded by his people as having smashed me. Now at the end of twenty years, I am told from Rome that I am guilty of the late Definition by my work on Development, so orthodox has it been found in principle, and on the other side Bampton Lectures have been preached, I believe, allowing that principle, the Guardian acknowledges the principle as necessary, and the Scotch Editors of Dorner’s great work on our Lord’s Person, cautioning of course the world against me, admit that development of doctrine is a historical fact. I shall not live another 20 years, but, as I waited patiently as regards my former work for ‘Time to be the Father of Truth’, so now I leave the judgment between Fr. Harper and me to the sure future.

In the case of the definition of 1854, even if Catholics had once doubted the doctrine, by the time of the definition it was certainly the general belief throughout the Catholic world, and even at the time of St. Thomas it was a common belief among Catholics. But in the case of papal infallibility, there was significantly more disagreement, at least about the details of the idea, even in the nineteenth century. Thus, when Newman says, “I am told from Rome that I am guilty of the late Definition,” the implication is that someone told him that without his theory of the development of doctrine, the council would not have agreed to make the definition.