"Therefore watch yourselves very carefully. Since you saw no form on the day that the LORD spoke to you at Horeb out of the midst of the fire, beware lest you act corruptly by making a carved image for yourselves, in the form of any figure..." (Deuteronomy 4:15f ESV)
"God did not show them a visible symbol of himself, but spoke to them; therefore they are not now to seek visible symbols of God, but simply to obey his Word. If it be said that Moses was afraid of the Israelites borrowing designs for images from the idolatrous nations around them, our reply is that undoubtedly he was, and this is exactly the point: all manmade images of God, whether molten or mental, are really borrowing from the stock-in-trade of a sinful and ungodly world, and are bound therefore to be out of accord with God's own holy Word. To make an image of God is to take one's thoughts of him from a human source, rather than from God himself; and this is precisely what is wrong with image-making."
(--J. I. Packer, Knowing God: 20th Anniversary Edition, p. 49)