Objection 1. It seems that sacred doctrine is not one science; for according to the Philosopher (Poster. i) "that science is one which treats only of one class of subjects." But the creator and the creature, both of whom are treated of in sacred doctrine, cannot be grouped together under one class of subjects. Therefore sacred doctrine is not one science.
Objection 2. Further, in sacred doctrine we treat of angels, corporeal creatures and human morality. But these belong to separate philosophical sciences. Therefore sacred doctrine cannot be one science.
On the contrary, Holy Scripture speaks of it as one science: "Wisdom gave him the knowledge [scientiam] of holy things" (Wisdom 10:10).
I answer that, Sacred doctrine is one science. The unity of a faculty or habit is to be gauged by its object, not indeed, in its material aspect, but as regards the precise formality under which it is an object. For example, man, ass, stone agree in the one precise formality of being colored; and color is the formal object of sight. Therefore, because Sacred Scripture considers things precisely under the formality of being divinely revealed, whatever has been divinely revealed possesses the one precise formality of the object of this science; and therefore is included under sacred doctrine as under one science.
Reply to Objection 1. Sacred doctrine does not treat of God and creatures equally, but of God primarily, and of creatures only so far as they are referable to God as their beginning or end. Hence the unity of this science is not impaired.
Reply to Objection 2. Nothing prevents inferior faculties or habits from being differentiated by something which falls under a higher faculty or habit as well; because the higher faculty or habit regards the object in its more universal formality, as the object of the "common sense" is whatever affects the senses, including, therefore, whatever is visible or audible. Hence the "common sense", although one faculty, extends to all the objects of the five senses. Similarly, objects which are the subject-matter of different philosophical sciences can yet be treated of by this one single sacred science under one aspect precisely so far as they can be included in revelation. So that in this way, sacred doctrine bears, as it were, the stamp of the divine science which is one and simple, yet extends to everything.
Source: Summa Theologica (New Advent)