St. Thomas says that Sacred Doctrine is a science. By this he does not mean something akin to modern science and its focus upon experiment; he means what Aristotle understood by the idea of science: “an organized body of systematically arranged information” (R. J. Hankinson in Barnes, ed. The Cambridge Companion to Aristotle, 109). “To have scientific knowledge, then, is to have explanatory understanding: not merely to ‘know’ a fact incidentally, to be able to assent to something which is true, but to know why it is a fact” (ibid., 110). Such a science proceeds primarily by way of demonstrations from certain first principles: either such as are self-evident, or such as are established by some other science. [Consequently what goes by the name of knowledge in common conversation today doesn’t pass muster for Aristotle or Aquinas as anything other than mere opinion…But I digress.]
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. [ST I Q1 A2]
One thing that seems worth noting here is that with this view of what a science is, and because he considers sacred doctrine to be a science, it seems doubtless that St. Thomas doesn’t intend to be offering what he considers to be mere opinions, but truths that are no less certain than those of any other science, since they are demonstrations following from first principles.