Objection 1. It seems that God is not the object of this science. For in every science, the nature of its object is presupposed. But this science cannot presuppose the essence of God, for Damascene says (De Fide Orth. i, iv): "It is impossible to define the essence of God." Therefore God is not the object of this science.
Objection 2. Further, whatever conclusions are reached in any science must be comprehended under the object of the science. But in Holy Writ we reach conclusions not only concerning God, but concerning many other things, such as creatures and human morality. Therefore God is not the object of this science.
On the contrary, The object of the science is that of which it principally treats. But in this science, the treatment is mainly about God; for it is called theology, as treating of God. Therefore God is the object of this science.
I answer that, God is the object of this science. The relation between a science and its object is the same as that between a habit or faculty and its object. Now properly speaking, the object of a faculty or habit is the thing under the aspect of which all things are referred to that faculty or habit, as man and stone are referred to the faculty of sight in that they are colored. Hence colored things are the proper objects of sight. But 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. Hence it follows that God is in very truth the object of this science. This is clear also from the principles of this science, namely, the articles of faith, for faith is about God. The object of the principles and of the whole science must be the same, since the whole science is contained virtually in its principles. Some, however, looking to what is treated of in this science, and not to the aspect under which it is treated, have asserted the object of this science to be something other than God — that is, either things and signs; or the works of salvation; or the whole Christ, as the head and members. Of all these things, in truth, we treat in this science, but so far as they have reference to God.
Reply to Objection 1. Although we cannot know in what consists the essence of God, nevertheless in this science we make use of His effects, either of nature or of grace, in place of a definition, in regard to whatever is treated of in this science concerning God; even as in some philosophical sciences we demonstrate something about a cause from its effect, by taking the effect in place of a definition of the cause.
Reply to Objection 2. Whatever other conclusions are reached in this sacred science are comprehended under God, not as parts or species or accidents but as in some way related to Him.
Source: Summa Theologica (New Advent)