St. Thomas addresses the question of doctrinal development in the Summa Theologica, II-II Q1 A7 ("Whether the articles of faith have increased in course of time?"). Before getting to the quotation from ST, it might be helpful for some of my readers to quickly review the structure of the book. St. Thomas addresses theological topics by first presenting a series of objections to the position that he intends to defend. This is followed by the sed contra ("on the contrary" in the translation below) in which he briefly states his view, supporting it with a quotation from some Scriptural or theological authority. After this, he explains his view to some degree, and then answers each of the objections.
(As an aside, this structure is one reason why I love ST: it's clear and methodical, and St. Thomas does his best (which is pretty devastating) to deal with every objection he either knows of or can imagine. It's an incredible performance. His consistency through 3000+ pages is staggering. But I digress.)
Hopefully this summary of the structure of the book will make the following snippet a bit more clear; if you visit the source you can see the whole thing in context.
Objection 1. It would seem that the articles of faith have not increased in course of time. Because, as the Apostle says (Hebrews 11:1), "faith is the substance of things to be hoped for." Now the same things are to be hoped for at all times. Therefore, at all times, the same things are to be believed.It seems pretty clear to me that there is nothing substantively different in what St. Thomas says here from Newman's formulation of the matter in his Essay on the Development of Christian Doctrine.
Objection 2. [Etc.]
On the contrary, Gregory says (Hom. xvi in Ezech.) that "the knowledge of the holy fathers increased as time went on . . . and the nearer they were to Our Savior's coming, the more fully did they received the mysteries of salvation"...[A]ll the articles are contained implicitly in certain primary matters of faith, such as God's existence, and His providence over the salvation of man, according to Heb. 11: "He that cometh to God, must believe that He is, and is a rewarder to them that seek Him." For the existence of God includes all that we believe to exist in God eternally, and in these our happiness consists; while belief in His providence includes all those things which God dispenses in time, for man's salvation, and which are the way to that happiness: and in this way, again, some of those articles which follow from these are contained in others: thus faith in the Redemption of mankind includes belief in the Incarnation of Christ, His Passion and so forth.
Accordingly we must conclude that, as regards the substance of the articles of faith, they have not received any increase as time went on: since whatever those who lived later have believed, was contained, albeit implicitly, in the faith of those Fathers who preceded them. But there was an increase in the number of articles believed explicitly, since to those who lived in later times some were known explicitly which were not known explicitly by those who lived before them. Hence the Lord said to Moses (Exodus 6:2-3): "I am the God of Abraham, the God of Isaac, the God of Jacob [Vulg.: 'I am the Lord that appeared to Abraham, to Isaac, and to Jacob'] . . . and My name Adonai I did not show them": David also said (Psalm 118:100): "I have had understanding above ancients": and the Apostle says (Ephesians 3:5) that the mystery of Christ, "in other generations was not known, as it is now revealed to His holy apostles and prophets" (emphasis added).
Getting back to St. Thomas, though, we can see that there is an acknowledgment of doctrinal development as far back as St. Gregory the Great in the sixth century, and (of course) a consideration of it by Aquinas in the 13th century. So it's just plain wrong to say that the idea of doctrinal development is something new, and pointless to try and beat Catholics over the head with it as though it were a "novelty." It's not.
It's probably worth pointing out, too, that this ought to be entirely non-controversial. Any reasonable-minded Protestant is going to have to concede that there are things even in the Bible that are implicit first and then explicit later: acorn to tree, as it were. See St. Thomas' examples above, but the doctrine of the Trinity is a great example too. In the OT, the singular focus with respect to the Godhead is pre-eminently on display in Deuteronomy 6:4: "Hear, O Israel! The LORD our God, the LORD is one!" Sure, there are some obscure hints at more than this, but it doesn't become explicit until the New Testament. In short, doctrinal development is an inevitable and healthy consequence of human interaction with God's revelation. The Church has come to understand it better over the millennia.