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).

John Calvin on the Origin of Idols

"In regards to the origin of idols . . . they originated with those who bestowed this honor on the dead, from a superstitious regard to their memory.  I admit that this perverse practice is of very high antiquity, and I deny not that it was a kind of torch by which the infatuated proneness of mankind to idolatry was kindled into a greater blaze.  I do not, however, admit that it was the first origin of the practice.  That idols were in use before the prevalence of that ambitious consecration of the images of the dead, frequently adverted to by profane writers, is evident from the words of Moses (Gen 31:19).  When he relates that Rachel stole her father's images, he speaks of the use of idols as a common vice.  Hence we may infer, that the human mind is, so to speak, a perpetual forge of idols. . . The human mind, stuffed as it is with presumptuous rashness, dares to imagine a God suited to its own capacity; as it labors under dullness, no, is sunk in the grossest ignorance, it substitutes vanity and an empty phantom in the place of God.  To these evils another is added.  The God whom man has thus conceived inwardly he attempts to embody outwardly.  The mind, in this way, conceives the idol, and the hand gives it birth.  That idolatry has its origin in the idea which men have, that God is not present with them unless his presence is carnally exhibited, appears from the example of the Israelites: 'Up,' said they, 'make us gods, which shall go before us; for as for this Moses, the man that brought us up out of the land of Egypt, we [know] not what is become of him' (Exod 32:1).  They knew, indeed, that there was a God whose mighty power they had experienced in so many miracles, but they had no confidence of his being near to them, if they did not with their eyes behold a [physical] symbol of his presence, as [a confirmation] to his actual government.  They desired, therefore, to be assured by the image which went before them, that they were journeying under divine guidance.  And daily experience shows, that the flesh is always restless until it has obtained some figment like itself, with which it may vainly [comfort] itself as a representation of God" (Institutes of the Christian Religion, John Calvin).