Newman's work on the development of doctrine took his lead from St. Vincent of Lerins in his Commonitorium or Commonitory which means something like "Remembrance-er."  It was a work to refresh his memory as to what distinguishes true doctrine from false. In this work he writes what comes to be known as the Vincentian Canon, a measurement by which we might determine truth from falsehood, namely, "Quod semper, quod ubique, quod ab omnibus." We should believe that which has been believed everywhere, always, and by all. Put another way, the tests of truth are universality, antiquity, and consent. This work is important because the charge against the Catholic Church is that they add to pure religion. What you find in the Commonitory is that St. Vincent insists on preservation, not innovation. He is conservative in that regard. It is the heretics who are charged with novelty. The schismatics are those who introduce new doctrine. The "changes" of the Catholic Church are the genuine developments to be expected from a living thing. It is "progress," not "alteration." He writes, "The growth of religion in the soul must be analogous to the growth of the body, which, though in process of years it is developed and attains its full size, yet remains still the same ... An infant's limbs are small, a young man's large, yet the infant and the young man are the same. Men when full grown have the same number of joints that they had when children; and if there be any [thing] to which maturer age has given birth these were already present in embryo, so that nothing new is produced in them when old which was not already latent in them when children. This, then, is undoubtedly the true and legitimate rule of progress, this is the established and most beautiful order of growth, that mature age ever develops in the man those parts and forms which the wisdom of the Creator had already framed beforehand in the infant" (XXIII). The constant chorus throughout is the contrast of "sacred antiquity" against "profane novelty." This is a great primer in the development of Christian doctrine. Highly recommended to all.