It is the office of the wise man to dispose things in order, and this he does in view of the end or final cause of these things. (SGC, I,1). Theology - which, more than any other science, is wisdom - has for its end or final cause the salvation of humanity: "It was necessary for man's salvation that there should be a knowledge revealed by God..." (ST Ia, q.1, a.1). Man's salvation consists in the knowledge of God: "For this is eternal life, that they might know Thee, the only true God..." (John 17:3). It is striking to me that St. Thomas gives human salvation as the purpose of theology itself, which is a science. It seems that St. Thomas must therefore closely associate the study of theology with the Christian life itself - that is, theology has something to do with living well, since it has for its end the eternal beatitude of man.
But theology does not only end with the beatific vision, the knowledge of God, it also begins with it. Every lower science, St. Thomas tells us (Ia, q.1, a.2), receives its principles from a higher science, as music receives its principles from mathematics. In each science, the principles are accepted by a kind of faith. In theology, these principles are none other than the articles of faith, which are received from "the science of God and the blessed." Beatific vision - God's knowledge of Himself, the participation of the blessed in this divine knowing - is the source and beginning of sacred theology. This is the structure of exitus-reditus: procession and return. The perfection of things consists in their return to the first principle from which they proceeded in the first place.
Theology is thus an essentially nostalgic affair: it seeks to return home, to its beginning, to the beginning of all things, indeed. God is called the subject of this science, even though other things are studied in theology, because it is He who is the principle and end of all things: things are only considered, in theology, inasmuch as they refer to Him as their principle and end. Moreover, the very division of theology, in the Summa, according to Thomas, is structure according to this conception: 1) God in Himself (in which we also consider God as Creator, i.e. beginning of all things); 2) man's advance and return to God (in which, accordingly, we consider God as the end of all things); and 3) Christ, who, as man, is our way to God. (Ia, q.2, prologue.)