The Essence of Christianity
The book is often considered a classic of humanism and the author's magnum opus. Karl Marx and Friedrich Engels were strongly influenced by the book, although they criticised Feuerbach for his inconsistent espousal of materialism. Feuerbach's theory of alienation would later be used by Marx in his theory of alienation. Max Stirner directed his The Ego And Its Own against it. Rather than simply a polemic, Stirner's work uses Feuerbach's idea of God as a human abstraction as the basis of his critique of Feuerbach. Although Stirner's book is often not understood within its context, it is structured as a response to Feuerbach.
"In the consciousness of the infinite, the conscious subject has for his object the infinity of his own nature" 
Feuerbach's theme was a derivation of Hegel's speculative theology in which the Creation remains a part of the Creator, while the Creator remains greater than the Creation. When the student Feuerbach presented his own theory to professor Hegel, Hegel refused to reply positively to it.
In Part I of his book, Feuerbach developed what he calls the "true or anthropological essence of religion." Treating of God in his various aspects "as a being of the understanding," "as a moral being or law," "as love" and so on. Feuerbach talks of how man is equally a conscious being, more so than God because man has placed upon God the ability of understanding. Man contemplates many things and in doing so he becomes acquainted with himself. Feuerbach shows that in every aspect God corresponds to some feature or need of human nature. "If man is to find contentment in God," he claims, "he must find himself in God."
Thus God is nothing else than man: he is, so to speak, the outward projection of man's inward nature. This projection is dubbed as a chimera by Feuerbach, that God and the idea of a higher being is dependent upon the aspect of benevolence. Feuerbach states that, "a God who is not benevolent, not just, not wise, is no God", and continues to say that qualities are not suddenly denoted as divine because of their godly association. The qualities themselves are divine therefore making God divine, indicating that man is capable of understanding and applying meanings of divinity to religion and not that religion makes a man divine.
The force of this attraction to religion though, giving divinity to a figure like God, is explained by Feuerbach as God is a being that acts throughout man in all forms. God, "is the principle of [man's] salvation, of [man's] good dispositions and actions, consequently [man's] own good principle and nature". It appeals to man to give qualities to the idol of their religion because without these qualities a figure such as God would become merely an object, its importance would become obsolete, there would no longer be a feeling of an existence for God. Therefore, Feuerbach says, when man removes all qualities from God, "God is no longer anything more to him than a negative being". Additionally, because man is imaginative, God is given traits and there holds the appeal. God is a part of man through the invention of a God. Equally though, man is repulsed by God because, "God alone is the being who acts of himself".
In part 2 he discusses the "false or theological essence of religion," i.e. the view which regards God as having a separate existence over against man. Hence arise various mistaken beliefs, such as the belief in revelation which he believes not only injures the moral sense, but also "poisons, nay destroys, the divinest feeling in man, the sense of truth," and the belief in sacraments such as the Lord's Supper, which is to him a piece of religious materialism of which "the necessary consequences are superstition and immorality."
Part 2 comes to a crux though by seemingly retracting previous statements. Feuerbach claims that God's only action is, “the moral and eternal salvation of man: thus man has in fact no other aim than himself,” because man's actions are placed upon God. Feuerbach also contradicts himself by claiming that man gives up his personality and places it upon God who in turn is a selfish being. This selfishness turns onto man and projects man to be wicked and corrupt, that they are, “incapable of good,” and it is only God that is good, “the Good Being.” In this way Feuerbach detracts from many of his earlier assertions while showing the alienation that takes place in man by worshipping God. Feuerbach affirms that goodness is, “personified as God,” turning God into an object because if God was anything but an object nothing would need to be personified on him. The aspect of objects having previously been discussed; in that man contemplates objects and that objects themselves give conception of what externalizes man. Therefore if God is good so then should be man because God is merely an externalization of man because God is an object. However religion would show that man is inherently corrupt. Feuerbach tries to lessen his inconsistency by asking if it were possible if, “I could perceive the beauty of a fine picture if my mind were aesthetically an absolute piece of perversion?” Through Feuerbach’s reasoning it would not be possible, but it is possible, and he later states that man is capable of finding beauty.
A caustic criticism of Feuerbach was delivered in 1844 by Max Stirner. In his book Der Einzige und sein Eigentum (The Ego and Its Own) he attacked Feuerbach as inconsistent in his atheism. The pertinent portions of the books, Feuerbach's reply, and Stirner's counter-reply form an instructive polemics. (see External Links)
- (1841) First. Das Wesen des Christenthums. Leipzig: Otto Wigand.
- (1843) Second. Das Wesen des Christenthums. Leipzig: Otto Wigand.
- (1848) Second. Das Wesen des Christenthums. Leipzig: Otto Wigand. Google (NYPL)
- (1849) Third. Ludwig Feuerbach's sämmtliche Werke. Volume 7. Leipzig: Otto Wigand. Google (Oxford)
- (1883) Fourth. Das Wesen des Christentums. Leipzig: Otto Wigand.
English (Translated by Marian Evans).
- (1854) First. The Essence of Christianity. London: John Chapman. IA (St. Mary's)
- (1881) Second. The Essence of Christianity. London: Trübner & Co. Google (Oxford)
- (1893) Third. The Essence of Christianity. London: Kegan Paul, Trench, Trübner & Co.
French (Translated by Joseph Roy)
- (1864) Essence du Christianisme. Paris: Librairie Internationale. Google (Ghent)
See also 
- http://www.marxists.org/glossary/people/f/e.htm Feuerbach, Ludwig
- The Essence of Christianity at the Feuerbach Internet Archive.
- Das Wesen des Christentums at Zeno.org. (German)
- The Essence of Christianity entry in the Stanford Encyclopedia of Philosophy
- Ludwig Feuerbach Internet Archive at Marxists Internet Archive.
- Ludwig Feuerbach: "The Essence Of Christianity" in Relation to (Max Stirner's) "The Ego And Its Own"
- Max Stirner: Stirner's Critics Reply to Feuerbach