A few quotes from the above mentioned authors:
'Religious distress is at the same time the expression of real distress and 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 a spiritless situation. It is the opium of the people. The abolition of religion as the illusory happiness of the people is required for their real happiness. The demand to give up the illusion about its condition is the demand to give up a condition which needs illusions.'
Karl Marx, Critique of Hegel’s Philosophy of Right
'Christianity was from the beginning, essentially and fundamentally, life's nausea and disgust with life, merely concealed behind, masked by, dressed up as, faith in "another" or "better" life.'
from Nietzsche's The Birth of Tragedy, p.23, Walter Kaufmann transl.
'The everyday Christian. -- If the Christian dogmas of a revengeful God, universal sinfulness, election by divine grace and the danger of eternal damnation were true, it would be a sign of weak-mindedness and lack of character not to become a priest, apostle or hermit and, in fear and trembling, to work solely on one's own salvation; it would be senseless to lose sight of ones eternal advantage for the sake of temporal comfort. If we may assume that these things are at any rate believed true, then the everyday Christian cuts a miserable figure; he is a man who really cannot count to three, and who precisely on account of his spiritual imbecility does not deserve to be punished so harshly as Christianity promises to punish him.'
from Nietzsche's Human, all too Human, s.116, R.J. Hollingdale transl.
'Christianity came into existence in order to lighten the heart; but now it has first to burden the heart so as afterwards to be able to lighten it. Consequently it shall perish.'
-- Friedrich Nietzsche, Human, all too Human, page 119, R J Hollingdale, translator
More found at
http://www.positiveatheism.org/hist/.../nietzsche.htm