Here is a short quote from Berkhof’s Systematic Theology on how we learn about God.

The only proper way to obtain perfectly reliable knowledge of the divine attributes is by the study of God’s self-revelation in Scripture. It is true that we can acquire some knowledge of the greatness and power, the wisdom and goodness of God through the study of nature, but for an adequate conception of even these attributes it will be necessary to turn to the Word of God. In the theology of revelation we seek to learn from the Word of God which are the attributes of the Divine Being. Man does not elicit knowledge from God as he does from other objects of study, but God conveys knowledge of Himself to man, a knowledge which man can only accept and appropriate. For the appropriation and understanding of this revealed knowledge it is, of course, of the greatest importance that man is created in the image of God, and therefore finds helpful analogies in his own life. In distinction from the a priori method of the Scholastics, who deduced the attributes from the idea of a perfect Being, this method may be called a posteriori, since it takes its starting point, not in an abstract perfect Being, but in the fulness of the divine self-revelation, and in the light of this seeks to know the Divine Being.