I agree with this and would like also to add the fact that the Apostles were a foundation to be built upon. He is not suggesting new revelation, hence the title “Development.”
p.s. chypmonk, earlier you suggested Cardinal Newman was going to be “canonized,” but I understand him to receive “beatification,” i.e. called “Blessed” and not “Saint” (yet). No?
slip of the “tongue”

should have said “beatification”
He is saying that there are new doctrines being created through time.
“The writer considers the growth of the doctrine [of Purgatory] an instance of the action of private judgment; whereas I should now call it an instance of the mind of the church **working out dogmatic truth from implicit feelings **, under secret supernatural guidance.”(pg.417)
St. Pius X again condemns these in his syllabus of Modernist Errors --Lamentabili Sane, July 3, 1907:
“22. The dogmas the Church holds out as revealed are not truths which have fallen from heaven. They are an interpretation of religious facts which the human mind has acquired by laborious effort.” – condemned
"The church went forth from the world in haste , as the Israelites from Egypt, ‘with their dough before it was leavened, their kneading-troughs being bound up in their clothes upon their shoulders.’…”(pg. 107)
“…Thus we see how, as time went on, the doctrine of Purgatory was opened upon the apprehension of the church , as a portion or form of penance due for sins committed after baptism: and thus the belief in **this doctrine and the practice of infant baptism **would grow into general reception together.”(pg. 417)
St. Pius X again condemns these in his syllabus of Modernist Errors --Lamentabili Sane, July 3, 1907— Infant Baptism:
#43. The practice of administering
Baptism to infants was a disciplinary evolution, which became one of the causes why the Sacrament was divided into two, namely, Baptism and Penance. – condemned
He is saying that the the apostles didn’t believe in Pugatory it was only a feeling.
Again with says the same of Original Sin:
Thus St. Justin or St. Irenaeus might be without any digested idea of Purgatory, or Original Sin,
yet have an intense feeling , which they had not defined or located, both of the fault of our first nature and of the liabilities of our nature regenerate.” (pg.83)
But the Catholic Encyclopedia says the Church Father had more than a feeling:
“It is not true that the doctrine of original sin does not appear in the works of the pre-Augustinian Fathers. **On the contrary, their testimony is found in special works on the subject. **Nor can it be said, as Harnack maintains, that St. Augustine himself acknowledges the absence of this doctrine in the writings of the Fathers. St. Augustine invokes the testimony of eleven Fathers, Greek as well as Latin (Contra Jul., II, x, 33). Baseless also is the assertion that before St. Augustine this doctrine was unknown to the Jews and to the Christians; as we have already shown, it was taught by St. Paul. It is found in the fourth Book of Esdras, a work written by a Jew in the first century after Christ and widely read by the Christians. This book represents Adam as the author of the fall of the human race (vii, 48), as having transmitted to all his posterity the permanent infirmity, the malignity, the bad seed of sin (iii, 21, 22; iv, 30). Protestants themselves admit the doctrine of original sin in this book and others of the same period (see Sanday, “The International Critical Commentary: Romans”, 134, 137; Hastings, “A Dictionary of the Bible”, I, 841). It is therefore impossible to make St. Augustine, who is of a much later date, the inventor of original sin.
That this doctrine existed in Christian tradition before St. Augustine’s time is shown by the practice of the Church in the baptism of children… Catholics argued, too, from the ceremonies of baptism, which suppose the child to be under the power of evil, i.e., exorcisms, abjuration of Satan made by the sponsor in the name of the child [Augustine, loc. cit., xxxiv, 63; Denz., n. 140 (96)].” [
CATHOLIC ENCYCLOPEDIA: Original Sin
](
http://www.newadvent.org/cathen/11312a.htm#IV)
and what of St. Paul: "Wherefore as by one man sin entered into this world, and by sin death; and so death passed upon all men, in whom all have sinned. " (Rom 5:12)