When St. Thomas argues in the Summa Theologiae that sacred doctrine is a science, he does not have in mind something that’s much like modern science or the scientific method. What he means is something that affords greater certainty than modern science may ever provide. “[E]very science proceeds from self-evident principles” and “The principles of any science are either in themselves self-evident, or reducible to the conclusions of a higher science” (I q.1 a.2). Starting from whatever its principles may be, such sciences as he has in view proceed by way of demonstration, drawing conclusions that constitute their bodies of knowledge. To the extent that this knowledge is the fruit of such demonstration, it’s reasonable to call it certain (and in fact this sort of thing is what Aquinas—following Aristotle—has in mind when he talks about knowledge; in his view most of what we call “knowledge” nowadays would be nothing more than a more or less justified opinion or belief).
Sacred doctrine is a science. We must bear in mind that there are two kinds of sciences. There are some which proceed from a principle known by the natural light of intelligence, such as arithmetic and geometry and the like. There are some which proceed from principles known by the light of a higher science: thus the science of perspective proceeds from principles established by geometry, and music from principles established by arithmetic. So it is that sacred doctrine is a science because it proceeds from principles established by the light of a higher science, namely, the science of God and the blessed. Hence, just as the musician accepts on authority the principles taught him by the mathematician, so sacred science is established on principles revealed by God. [ibid.]
He says that divine revelation is the starting point for sacred doctrine. In other words sacred doctrine doesn’t consist in things merely imagined by anyone, but rather it consists in the truths revealed by God and in those truths that we may draw out by way of demonstration from “the principles revealed by God.” This seems to be consistent with doctrinal development, something that St. Thomas unambiguously affirmed:
The articles of faith stand in the same relation to the doctrine of faith, as self-evident principles to a teaching based on natural reason. Among these principles there is a certain order, so that some are contained implicitly in others; thus all principles are reduced, as to their first principle, to this one: “The same thing cannot be affirmed and denied at the same time,” as the Philosopher states (Metaph. iv, text. 9). On like manner all 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 Hebrews 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…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.” [ST II-II q.1 a.7]