According to its etymology, Theology is the science concerning God. Other definitions either are misleading, or, when closely examined, are found to lead to the same result. . . . From this definition of Theology as the science concerning God follows the necessity of its being based on revelation. In scientifically dealing with impersonal objects we ourselves take the first step; they are passive, we are active; we handle them, examine them, experiment with them. But in regard to a spiritual, personal being this is different. Only in so far as such a being choose to open up itself can we come to know it. All spiritual life is by its very nature a hidden life, a life shut up in itself. Such a life we can know only through revelation. If this be true as between man and man, how much more must it be so as between God and man. The principle involved has been strikingly formulated by Paul: ‘For who among men knoweth the things of a man, save the spirit of the man which is in him? even so the things of God none knoweth, save the Spirit of God’ [1 Cor. 2.11].
Vos, Biblical Theology, 3.