I have often wondered why theology covers so many subjects. Aquinas explains the boundaries of theology: it is always ultimately about God, and it rests on the Bible. The following quotes are from this Kindle version, pages 45-48.
Theology Studies God
“For it is called theology, as treating of God. Therefore God is the object of this science.”
Theology Studies Creation Only as it Relates to God
In sacred science, all things are treated of under the aspect of God: either because they are God Himself or because they refer to God as their beginning and end.
Scripture is the Sole Infallible Rule of Faith
Sacred doctrine… properly uses the authority of the canonical Scriptures as an incontrovertible proof, and the authority of the doctors of the Church as one that may properly be used, yet merely as probable. For our faith rests upon the revelation made to the apostles and prophets who wrote the canonical books, and not on the revelations (if any such there are) made to other doctors. Hence Augustine says (Epis. ad Hieron. xix, 1): “Only those books of Scripture which are called canonical have I learned to hold in such honor as to believe their authors have not erred in any way in writing them. But other authors I so read as not to deem everything in their works to be true, merely on account of their having so thought and written, whatever may have been their holiness and learning.”