John Calvin on Why God Forbids Images of Him

"When once men imagined that they beheld God in images, they also worshiped him as being there.  At length their eyes and minds becoming wholly engrossed by them, they began to grow more and more brutish, gazing and wondering as if some divinity were actually before them.  It hence appears that men do not fall away to the worship of images until they have [swallowed] some idea of a grosser description: not that they actually believe them to be gods, but the the power of divinity somehow or other resides in them.  Therefore, whether it be God or a creature that is imaged, the moment you fall prostrate before it in veneration, you are so far fascinated by superstition.  For this reason, the Lord not only forbade the erection of statues to himself, but also the consecration of titles and stones which might be set up for adoration.  For as soon as a visible form is given to God, his power also is supposed to be annexed to it.  So stupid are men, that wherever they figure God, there they fix him, and by necessary consequence proceed to adore him.  It makes no difference whether they worship the idol simply, or God in the idol; it is always idolatry when divine honors are paid to an idol, be the color what it may.  And because God wills not to be worshiped superstitiously, whatever is bestowed upon idols is so much robbed from him" (Institutes of the Christian Religion, John Calvin).