Chapter 4 of Knowing God is “The Only True God,” and it has to do with the prohibition in the Ten Commandments against making idols.
“Idolatry consists not only in the worship of false gods, but also in the worship of the true God by images” (quoted from Charles Hodge).
Just as it forbids us to manufacture molten images of God, so it forbids us to dream up mental images of Him… How often do we hear this sort of thing: ‘I like to think of God as the great Architect (or, Mathematician; or Artist).’ ‘I don’t think of God as a Judge; I like to think of Him simply as a Father.’ We know from experience how often remarks of this kind serve as the prelude to a denial of something that the Bible tells us about God.
We were made in His image, but we must not think of Him as existing in ours.
Thus it appears that the positive force of the second commandment is that it compels us to take our thoughts of God from His own holy Word.
Grace and Peace