John Calvin famously said "the human heart is a perpetual idol factory" (hominis ingenium perpetuam, ut ita loquar, esse idolorum fabricam) [Institutes I.11.8]. Calvin's Latin phrase from the definitive 1559 edition of his Institutes of the Christian Religion has received meaning that may be lost to Post-Enlightenment people. A modern translation as "the human condition is driven by idolatry" is closer to Calvin's intent, but is still lacking. Today, "idol" (idolorum) may positively refer to celebrity pop tarts, athletes, politicians or personal heroes, and "factory/fabricator" (fabricam) refers to semiconductor fabrication plants, or overseas sweatshops, and in conjunction "idol factory" (idolorum fabricam) refers to television series like American Idol or action figure manufacturing. In Calvin's 16th century sitz im leben, an "idol factory" referred to the prevalence of blacksmith workshops where figurines were handcrafted to venerate pagan gods (and were used in Christian veneration too). 

What are "idols"?

In the Institutes I.11.8, John Calvin explains that idols are handcrafted representations of gods, typically in the form of a picture or statue, and they specifically created to give an invisible deity a visible form. Calvin explains that idols replace the true invisible divine reality with a corrupted false visible physical reality. And so, for Calvin, idolatry occurs every time the truth about God is exchanged for a lie, because idolatry is the worship of a created reality, rather than the Creator (Romans 1:25).

And so, Calvin has harsh words against idolatry:

So it goes. Man's mind, full as it is of pride and boldness, dares to imagine a god according to its own capacity; as it sluggishly plods, indeed is overwhelmed with the crassest ignorance, it conceives an unreality and an empty appearance as God. To these evils a new wickedness joins itself, that man tries to express in his work the sort of God he has inwardly conceived. Therefore the mind begets an idol; the hand gives it birth. The example of the Israelites shows the origin of idolatry to be that men do not believe God is with them unless he shows himself physically present. "We know not," they said, "what has become of this Moses; make us gods who may go before us." [Ex. 32:1.] They knew, indeed, that this was God whose power they had experienced in very many miracles; but they did not trust that he was near them unless they could discern with their eyes a physical symbol of his countenance, which for them would be a testimony of the ruling God. Therefore they wished to recognize from an image going before them that God was the leader of their march. Daily experience teaches that flesh is always uneasy until it has obtained some figment like itself in which it may fondly find solace as in an image of God. In almost every age since the beginning of the world, men, "in order that they might obey this blind desire, have set up symbols in which they believed God appeared before their bodily eyes. [1]

Modern Criticism of Calvin's Concept of Idolatry

In the Institutes I.11.8, John Calvin argues that did not originate with the honoring of the dead as it says in the Wisdom of Solomon 14:15-16. Calvin held to a pre-Copernican Ptolemaic cosmology, and although he admits that history of the Pentateuch is incorrect at time, nevertheless Calvin still assumes that Genesis is an accommodation of history as it happened. For instance, Calvin argues that idolatry was a universal pathology since the beginning of human history, however he assumes Noah's flood was a historical event that temporarily halted idolatry, and this idolatry resumed shortly after the Flood, and Calvin appeals to Gen 31:19, Josh 24:2, and other verses as proofs. So unfortunately, Calvin's exegesis is not helpful for understanding origination of idolatry in human evolution, but Calvin is right to conclude idolatry is bound up in human nature and is integrated with the human condition in the general sense of exchanging the true knowledge of god for something tangible (like eating the fruit in the Garden of Eden) in the first theodicy of Genesis 3. (FYI, the second theodicy is in Genesis 6, and is quite different).

Next, what is held in the book of Wisdom concerning the origin of idols is received virtually by public consent: that the originators of idols were those who conferred this honor on the dead [Wisdom of Solomon 14:15-16], and thus superstitiously worshiped their memory. Of course, I admit that this perverse custom was very ancient, nor do I deny that it was a torch with which to fire men's mad dash into idolatry all the more; yet I do not concede that this was the original source of the evil. For it appears from Moses that idols were in use before this eagerness to consecrate images of the dead prevailed, which is frequently mentioned by secular writers. When he relates that Rachel stole her father's idols [Gen. 31:19], he is speaking of a vice that was common. From this we may gather that man's nature, so to speak, is a perpetual factory of idols. After the Flood there was a sort of rebirth of the world, but not many years passed by before men were fashioning gods according to their pleasure. And it is believable that while the holy patriarch was still living his descendants were giving themselves over to idolatry, so that he discerned with his eyes (not without the bitterest pain) that the earth, whose corruptions God had recently purged by a most dreadful judgment, was polluted with idols. For as Joshua testifies [Josh. 24:2], Terah and Nahor were worshipers of false gods before the birth of Abraham. If Shem's offspring degenerated very rapidly, what are we to judge of Ham's descendants who had already been cursed in their father? [2]

So Calvin's correct assessment that idolatry is integrated with the human condition is correct, and helpful for any theology of sin (if we may have one at all!) He is also correct to acquit Ancestor Cult for introducing idolatry in the work. Unfortunately, Calvin was wrong to condemn Ancestor Cult, and perhaps if he had more knowledge of Eastern cultures and religions, then he would have revised his anathemas of it. (See Hans Kung's Christianity: Essence, History and Future for a better understanding of Ancestor Cult within a Christianity.) Additionally, in Calvin's defense, his definition of idolatry as replacing an invisible reality with a visible fantasy has a direct application to scientific naturalism, and anyone who limits knowledge to the observable evidence or rejects invisible metaphysical truths.

Conclusions on Idolatry

Calvin thoroughly rejected all forms of idolatry, and icon veneration, but he did not reject art. I recommend reading the Institutes I.11 entirely to understand Calvin's broader statement. Calvin is correct that the human condition is enslaved by idolatry (c.f. Rom 1:25), and that is a helpful pastoral message for anyone who is overwhelmed with the constant temptation to sin, or the pains of the human condition today. Calvin's prohibition on images in the church resulted in simplistic worship settings in Reformed churches, which I admire for the minimalism and reduced cost of building churches. Calvin's iconoclasm also bring Christianity in closer proximity and unity with the rejection of images by Judaism and Islam too. However, the negative cost is that Calvin's views have fuelled fanatical iconoclasm (smashers of images), and has resulted a rejection of the later ecumenical councils (such as Nicea II) that approved icon veneration, and has created unnecessary discord with branches of Christianity such as Catholicism and Orthodoxy that use icon veneration extensively.  Lastly, Calvin's definition of idolatry has a benefit for scientific ethics, because truth may not be limited to empirical evidence or the scientific method alone as in scientific naturalism. 

Sources:

1. John Calvin, Institutes of the Christian Religion. Ed. John T. McNeill. Trans. Ford Lewis. Battles. Philadelphia, PA: Westminster, 1960. p. 108. Print. [Institutes 1.11.8; References were reformatted and paragraph breaks were added for readability] 

2. Calvin. Ibid. pp. 107-8.

3. Header image contains an image of a heart, modified from a public source at wikipedia.org