"When what begins by being an image becomes in course of time a mere thing, we are justified in describing it as an idol," we are told in History, Guilt, and Habit (70). Idolatry is thus a "collective state of mind, which perceives all things and no images" (HGH 70). It is "the effective tendency to abstract the sense-content from the whole representation and seek that for its own sake, transmitting the admired image into a desired object" (SA 111).

Idolatry indicates that "within man the phenomena have gradually ceased to operate as compulsive natural processes and have become, instead, mere memory-images available for his own creative 'speech'. . ." (SA 127). Idolatry results when man begins to take his models--his representations--literally (SA 51). It is produced by what Coleridge called the "lethargy of custom" (RCA 15). By the time of Darwin, for example, the idolatry of alpha-thinking had "clothed [appearances] with the independence and extrinsicality of the unrepresented itself" (SA 62).

Barfield often refers to the following Old Testament discourse on idolatry (from the 135th Psalm):

    The idols of the heathen are silver and gold, the work of men's hands. they have mouths, but they speak not: eyes have they, but they see not; They have ears, but they hear not: neither is there any breath in their mouths. They that make them are like unto them: so is every one that trusteth in them. (quoted in SA 176)
And he offers his own commentary on it:
    What the Psalmist wrote of the old idols is true no less of the idols of the twentieth century. "They that make them are like unto them." The soul is in a manner all things, and the idols we create are built into the souls of our children; who learn more and more to think of themselves as objects among objects; who grow hollower and hollower. In the long run we shall not be able to save souls without saving the appearances, and it is an error fraught with the most terrible consequences to imagine that we shall. (SA 160-61)
In "The Evolution Complex" Barfield notes that "If you wake up to find yourself in a darkened room, I think there's a lot of point in finding out whether it is dark because there is no sunlight outside, or because someone closed the shutters yesterday" (EC 9).1 The shutters in his metaphor are idolatry.
    Opening the shutters is no simple matter however, for they are not easily grasped. Though they have taken on the appearance of matter, their origins are immaterial, and it to their spiritual origins that we must turn if we expect to escape "dead thinking" and see the light.
As Barfield reminds us in the final words of "Speech, Imagination and Reason," "The 'Satanic Mills,' which have arisen over England since Blake's time, will never be thrust down from their hideous tyranny, until those of which he actually sang--the dead thinking of Newton, Locke, and Hobbes--have been burst asunder from within" (RCA 83).
See in particular Saving the Appearances, passim and History, Guilt and Habit, passim.
1Compare, too, the quite similar metaphor concerning the discovery of idolatry from History, Guilt, and Habit
    I suppose the most important question for a prisoner is, whether or not there is any way of escaping. . . . It sounds as if it ought to be easy enough, where the prison in question is not made of steel and concrete but only of mental habit. But it is not. Remember it is not just my mental habit, or your mental habit. It is our mental habit. (HGH 72-73)