From the 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 chooses 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? ‘ (I Cor. 2:11). The inward hidden content of God’s mind can become the possession of man only through voluntary disclosure on God’s part. God must come to us before we can go to Him. But God is not a personal spiritual being in general. He is a Being infinitely exalted above our highest conception.
Geerardus Vos, Biblical Theology: Old and New Testaments, p.3-4