By the mid-1840s Karl Marx and Friedrich Engels had begun to use the term “ideology” in something approaching its modern meaning. They showed themselves aware of having broken through into a new area of understanding and encountered something other than the familiar operations of thought, but did not recognise the full significance of their own perception. In their work ideology remains secondary, an effect produced by class interests. They tried to use the concept as a political weapon, linking it with the false consciousness they ascribed to those who supported what they regarded as bourgeois movements and beliefs but not mentioning (and apparently not recognising) its effect upon what they termed the proletarian movement. At least, they did this at first; there are some grounds for thinking that their ideas were changing in later life.
Their original presentation appears in The German Ideology, written in 1845-6 (although not published until 1922), and here they speak without hesitation:
In direct contrast to German philosophy which descends from heaven to earth, here we ascend from earth to heaven. That is to say, we do not set out from what men say, imagine, conceive, nor from men as narrated, thought of, imagined, conceived, in order to arrive at men in the flesh. We set out from real, active men, and on the basis of their real life-process we demonstrate the development of the ideological reflexes and echoes of this life-process. The phantoms formed in the human brain are also, necessarily, sublimates of their material life-process, which is empirically verifiable and bound to material premises. Morality, religion, metaphysics, all the rest of ideology and their corresponding forms of consciousness, thus no longer retain the semblance of independence. They have no history, no development; but men, developing their material production and their material intercourse, alter, along with this their real existence, their thinking and the products of their thinking. Life is not determined by consciousness, but consciousness by life. 
Ideology consists of reflexes and echoes, it is bound to material processes, has no semblance of independence, no history, no development. Thinking and the products of thinking are not included in real existence. Consciousness does not determine life. This is brought into political focus in the Communist Manifesto of 1848 with the statement that because the proletarians are a class they will become a party: “this organisation of the proletarians into a class and consequently into a political party”  (Emphasis added).
At this period, from the mid to the late 1840s, Marx and Engels were seeing ideology as an epiphenomenon of processes in the material world, without life or growth of its own. This view carries the corollary that in order to understand ideology we should not study ideology itself but turn our attention to the processes in the real, material world which it echoes. Continuing adherence to it by Marxists today explains why their studies allegedly of ideology are seldom much more than accounts of class relations.
Marx and Engels themselves were less rigidly fixed in their thinking. In 1890 Engels (evidently without referring back to the German Ideology) was writing to Joseph Bloch:
according to the materialist conception of history, the ultimately determining element in history is the production and reproduction of real life. More than this neither Marx nor I have ever asserted. Hence if anybody twists this into saying that the economic element is the only determining one, he transforms that proposition into a meaningless, abstract, senseless phrase. The economic situation is the basis, but the various elements of the superstructure… political, juristic, philosophical theories, religious views and their further development in the systems of dogmas – also exercise their influence upon the course of the historical struggles and in many cases preponderate in determining their form.”  (Emphasis in the original).
The material, economic element is no longer the only determinant. Ideology is no longer seen as confined to reflexes and echoes, it has acquired history and development, it has become a part of real life, exercising a determining influence, sometimes even a prepondering one.
Three years later Engels had moved still farther in the same direction, writing to Franz Mailing, Marx’s biographer:
one more point is lacking, which, however, Marx and I always failed to stress enough in our writings and in regard to which we are all equally guilty. That is to say, we all laid, and were bound to lay, the main emphasis, in the first place, on the derivation of political, juridical, and other ideological notions, and of actions arising through the medium of these notions, from basic economic facts. But in so doing we neglected the formal side – the ways and means by which these notions etc. come about – for the sake of the content…
This aspect of the matter, which I can only indicate here, we have all, I think, neglected more than it deserves. It is the old story, form is always neglected at first for content. As I say, I have done that too, and the mistake has always struck me only later. 
The letter goes on to speak of “the fatuous notion of the ideologists, that because we deny an independent historical development to the various ideological spheres which play a part in history we also deny them any effect upon history” (Emphases in the original). 
The context of these last extracts shows Engels still holding that economic, material factors ultimately determine ideology, but his thinking here has moved far beyond the original presentation in the German Ideology. In 1845-6 ideology was a reflex, an echo, something apart from real life. Consciousness explicitly did not determine life. But fifty more years of thinking and experience have brought Engels to scorn as “fatuous” the notion that he and Marx might ever have held that ideology did not affect history.
The great barrier remained still to be crossed; Engels was still thinking of ideological processes as they appear in religion, politics, juristics and so on; his effort was to show that these were fundamentally determined by material, economic factors and not vice versa. He had not yet grasped the conception of ideology as an underlying process of which both economic and political (etc.) activities are appearances on the surface of social life. But he had advanced farther in that direction than most of those who call themselvesMarxiststoday.
1.Marx K. and Engels F. 1970. The German Ideology p. 47. London: Lawrence & Wishart.
2. ibid, 1948. The Communist Manifesto and the Last Hundred Years p. 69. London: Socialist Party of Great Britain.
3.Engels F. Letter to Joseph Bloch, in: Lewis S. Feuer, editor: 1969 Karl Marx and Friedrich Engels, Basic Writings on Politics and Philosophy, p. 436. London: Wm.Collins & Co. Ltd.
4. ibid, Letter to Franz Mehring. Ibid. p.446.
from Ideological Commentary 38, March 1989.