Sciences are not formed at haphazard, but according to an objective principle of division. As in general science is bound by its object and must let itself be shaped by reality; so likewise the classification of sciences, the relation of the various members in the body of universal knowledge, has to follow the great lines by which God has mapped out the immense field of the universe. The title of a certain amount of knowledge to be called a separate science depends on its reference to such a separate and specific object as is marked off by these God-drawn lines of distinction. We speak of a science of Biology, because God has made the phenomena of life distinct from those of inorganic being.
Now, from this point of view we must say that no science has a clearer title to separate existence than Theology. Between God as the Creator and all other things as created the distinction is absolute. There is not another such gulf within the universe. God, as distinct from the creature, is the only legitimate object of Theology.
It will be seen, however, on a moment's reflection, that Theology is not merely distinguished from the other sciences by its object, but that it also sustains an altogether unique relation to this object, for which no strict analogy can be found elsewhere. In all the other sciences man is the one who of himself takes the first step in approaching the objective world, in subjecting it to his scrutiny, in compelling it to submit to his experiments — in a word, man is the one who proceeds actively to make nature reveal her facts and her laws. In Theology this relation between the subject and object is reversed. Here it is God who takes the first step to approach man for the purpose of disclosing His nature, nay, who creates man in order that He may have a finite mind able to receive the knowledge of His infinite perfections.
In Theology the object, far from being passive, by the act of creation first posits the subject over against itself, and then as the living God proceeds to impart to this subject that to which of itself it would have no access. For "the things of God none knoweth, save the Spirit of God." Strictly speaking, therefore, we should say that not God in and for Himself, but God in so far as He has revealed Himself, is the object of Theology.
Geerhardus Vos, The Idea of Biblical Theology as a Science and as a Theological Discipline