I thought this short text from 1844 was interesting in relation to the Heidegger bit and some of the stuff that's come up in class with Feuerbach and The Essence of Christianity. (There's italics all over the original text so it may be use following the link and the bottom to read it.)
The profane existence of error is discredited after its heavenly oratio pro aris et focis has been disproved. Man, who looked for a superhuman being in the fantastic reality of heaven and found nothing there but the reflection of himself, will no longer be disposed to find but the semblance of himself, only an inhuman being, where he seeks and must seek his true reality.
The basis of irreligious criticism is: Man makes religion, religion does not make man. Religion is the self-consciousness and self-esteem of man who has either not yet found himself or has already lost himself again. But man is no abstract being encamped outside the world. Man is the world of man, the state, society. This state, this society, produce religion, an inverted world-consciousness, because they are an inverted world. Religion is the general theory of that world, its encyclopaedic compendium, its logic in a popular form, its spiritualistic point d'honneur, its enthusiasm, its moral sanction, its solemn complement, its universal source of consolation and justification. It is the fantastic realisation of the human essence because the human essence has no true reality. The struggle against religion is therefore indirectly a fight against the world of which religion is the spiritual aroma.
Religious distress is at the same time the expression of real distress and also the protest against real distress. Religion is the sigh of the oppressed creature, the heart of a heartless world, just as it is the spirit of spiritless conditions. It is the opium of the people.
To abolish religion as the illusory happiness of the people is to demand their real happiness. The demand to give up illusions about the existing state of affairs is the demand to give up a state of affairs which needs illusions. The criticism of religion is therefore in embryo the criticism of the vale of tears, the halo of which is religion.
Criticism has torn up the imaginary flowers from the chain not so that man shall wear the unadorned, bleak chain but so that he will shake off the chain and pluck the living flower. The criticism of religion disillusions man to make him think and act and shape his reality like a man who has been disillusioned and has come to reason, so that he will revolve round himself and therefore round his true sun. Religion is only the illusory sun which revolves round man as long as he does not revolve round himself.
The task of history, therefore, once the world beyond the truth has disappeared, is to establish the truth of this world. The immediate task of philosophy, which is at the service of history, once the holy form of human self-estrangement has been unmasked, is to unmask self-estrangement in its unholy forms. Thus the criticism of heaven turns into the criticism of the earth, the criticism of religion into the criticism of law and the criticism of theology into the criticism of politics.....
The weapon of criticism cannot, of course, replace criticism by weapons, material force must be overthrown by material force; but theory also becomes a material force as soon as it has gripped the masses. Theory is capable of gripping the masses as soon as it demonstrates ad hominem, and it demonstrates ad hominem as soon as it becomes radical. To be radical is to grasp the root of the matter. But for man the root is man himself. The evident proof of the radicalism of German theory, and hence of its practical energy, is that it proceeds from a resolute positive abolition of religion. The criticism of religion ends with the teaching that man is the highest being for man, hence with the categorical imperative to overthrow all relations in which man is a debased, enslaved forsaken, despicable being.....