Karl Marx, selection from Contribution to the Critique of Hegel's Philosophy of Right. (1843):

The basis of irreligious criticism is this: man makes religion; religion does not make man. Religion is indeed man's selfconsciousness and self-awareness so long as he has not found himself or has lost himself again. But man is not an abstract being, squatting outside the world. Man is the human world, the state, society. This state, this society, produce religion which is an inverted world consciousness, because they are an inverted world. Religion is the general theory of this world, its encyclopedic compendium, its logic in popular form, its spiritual point d'honneur, its enthusiasm, its moral sanction, its solemn complement, its general basis of consolation and justification. It is the fantastic realization
of the human being inasmuch as the human being possesses no true reality. The struggle against religion is, therefore, indirectly a struggle against that world whose spiritual aroma is religion.

Religious suffering is at the same time an expression of real suffering and a protest against real suffering. Religion is the sigh of the oppressed creature, the sentiment of a heartless world, and the soul of soulless conditions. It is the opium of the people.

The abolition of religion as the illusory happiness of men, is a demand for their real happiness. The call to abandon their illusions about their condition is a call to abandon a condition which requires illusions. The criticism of religion is, therefore, the embryonic criticism of this vale of tears of which religion is the halo.

Criticism has plucked the imaginary flowers from the chain, not in order that man shall bear the chain without caprice or consolation but so that be shall cast off the chain and pluck the living flower. The criticism of religion disillusions man so that he will think, act and fashion his reality as a man who has lost his illusions and regained his reason; so that be will revolve about himself as his own true sun. Religion is only the illusory sun about which man revolves so long as he does not revolve about himself.

It is the task of history, therefore, once the other-world of truth has vanished, to establish the truth of this world. The immediate task of philosophy, which is in the service of history, is to unmask human self-alienation in its secular form now that it has been unmasked in its sacred form. Thus the criticism of heaven is transformed into the criticism of earth, the criticism of religion into the criticism of law, and the criticism of theology into the criticism of politics.

 

Nietzsche, The Gay Science, from Aph. 357:

(he is referring to the philosopher Schopenhauer, but also indirectly to himself)

This is the locus of his whole integrity; unconditional and honest atheism is simply the presupposition of the way he poses his problem, being a triumph achieved finally and with great difficulty by the European conscience, being the most fateful act of two thousand years of discipline for truth that in the end forbids itself the lie in faith in God.

You see what it was that really triumphed over the Christian god: Christian morality itself, the concept of truthfulness that was understood ever more rigorously, the father confessor's refinement of the Christian conscience, translated and sublimated into a scientific conscience, into intellectual cleanliness at any price. Looking at nature as if it were proof of the goodness and governance of a god; interpreting history in honor of some divine reason, as a continual testimony of a moral world order and ultimate moral purposes; interpreting one's own experiences as pious people have long enough interpreted theirs, as if everything were providential, a hint, designed and ordained for the sake of the salvation of the soul—that is all over now, that has man's conscience against it, that is considered indecent and dishonest by every more refined conscience—mendaciousness, feminism, weakness, and cowardice. In this severity, if anywhere, we are good Europeans and heirs of Europe's longest and most courageous self-overcoming.